Dissection 5. Pelvis and Perineum
 

This guide to dissection replaces the assigned pages in Grant's Dissector. Its goals are: l ) to orient you and prepare you in advance for this dissection, and 2) to take you through the dissection in an orderly and logical way, highlighting important structural and functional relationships in the pelvis and perineum. The guide is divided into two parts: 1. A self-instructional module which should be used to acquaint you with the three-dimensional orientation of the region. 2. Instructions for carrying out the dissection of the pelvis and perineum.

Remember that, although you will only be responsible for dissecting one cadaver, you are expected to acquire a reasonable understanding of the pelvis and perineum in both sexes. You should therefore read through the orientation and instructions for dissection in both sexes. Make sure that you examine cadavers of both sexes when the dissections are complete.

Part 1. Orientation and guide to the anatomy of the pelvis and perineum: a self-instructional module.

You should work your way through the material contained in this self instructional module before you attempt the actual dissection of the pelvis and perineum. The best time for you to do this will be during the week before your assigned dissecting time, i.e. while the A-team is completing its dissection of the abdomen. If you attempt the dissection without completing the self-instructional module you will waste considerable time and effort, as well as subjecting yourself to a great deal of frustration.

Space and a relatively quiet and odor-free atmosphere for your work will be provided in Rooms 337 and 338 of the Anatomy-Physiology Building. You will need to have access to an articulated pelvis to complete this self-instructional module. You will also find it helpful to have string, tape, paper and scissors. These will be available in Rooms 337 and 338. The single os coxa and sacrum provided in your bone box may also be useful for orientation. Please remember to treat all skeletal materials and articulated pelves with care. DO NOT MAKE MARKS ON DISARTICULATED BONES OR ARTICULATED PELVES. You may, however, use paper, tape and string on the bones as long as they are carefully removed when you are finished with this exercise. HANDLE THE ARTICULATED PELVES MADE OF REAL BONE WITH EXTREME CARE. THESE ARE VERY FRAGILE.

Start by becoming familiar with the correct anatomical orientation and bony parts of an articulated pelvis. The orientation of the pelvis with the body in the upright position is with the anterior superior iliac spine and the superior part of the pubic symphysis in a vertical plane, as shown below.
 

Figure 1. Orientation of the pelvis in the upright position. View of the right side of the pelvis. (From: Grant's Atlas, Fig. 3.9.)

You can verify that this is the correct orientation by palpating these bony landmarks on yourself or another student while in the upright position, or by looking at an articulated skeleton.

Next, viewing the bony pelvis from the front (anterior view) (see Fig. 2 below), verify for yourself the following points:
 

Figure 2. Frontal view of bony pelvis. (Modified slightly from: Snell. Clinical Anatomy for Medical Students. Figure 5b, p. 260.)
 
Figure 3. Lateral view of right os coxa. (From: Snell, Clinical Anatomy for Medical Students. Fig. 12b, p. 266.)
 
Figure 4. (left) Ventral (anterior) view of the sacrum and coccyx. (Modified slightly from: Snell, Clinical Anatomy for Medical Students. Fig. 9a, p. 263).
Figure 5. (right) Dorsal (posterior) view of the sacrum and coccyx. (Modified slightly from Snell, Clinical Anatomy for Medical Students. Fig. 9b, p. 263).
 
Figure 6. Joints of the pelvis indicated on frontal view of bony pelvis. (Modified slightly from: Snell, Clinical Anatomy for Medical Students, Fig 5b, p. 260.)
 
Now that you are familiar with the bony composition and major joints of the pelvis, you should make sure you can readily identify the following bony landmarks on an articulated pelvis, since they will be important in discussing pelvic anatomy. Use Figs. 7-8 below for orientation.
  1. The pelvic inlet or pelvic brim: a bony rim made up of the sacral promontory, the iliopectineal line and the superior border of the svmphysis pubis.
  2. The pubic tubercle(s).
  3. The obturator foramina.
  4. The pubic arch (made up of a right and a left ramus).
  5. The ischial tuberosities: verify on yourself that this is the part you sit on.
  6. The ischial spines.
  7. The acetabulum. A Unshaped socket which articulates with the head of the femur (see Fig. 3).  
Figure 7. Frontal view of the bony pelvis indicating major bony landmarks 1-7 above. (Modified slightly from: Snell, Clinical Anatomy for Medical Students, Fig. 5a, p.260.)
  1. The pelvic outlet: A diamond-shaped area, best appreciated when viewed from below. The boundaries of the pelvic outlet are: the pubic symphysis, right and left rami of the pubic arch, the ischial tuberosities, the sacrotuberous ligaments (described below) and the tip of the coccyx.
  
Figure 8. Inferior view of the outlet of the bony pelvis. (Modified slightly from Woodburne, Essentials of Human Anatomy, Figure 375, p. 465)

In the living person (or the intact cadaver) there are 2 important ligaments which help to stabilize the bony pelvis:

  1. The sacrospinous ligament.
  2. The sacrotuberous ligament.
 
Figure 9 (left). Frontal view of the bony pelvis illustrating the disposition of the sacrotuberous and sacrospinous ligaments on the right side. (Modified slightly from: Snell, Clinical Anatomy for Medical Students, Fig. 5a, p.260.)
 
Figure 10 (right). Dorsal (posterior) view of the right side of the bony pelvis, showing the disposition of the sacrotuberous and sacrospinous ligaments. (Modified slightly from: Basmajian, Grant's Method of Anatomy, Fig. 17.21, p.215.)
 

Using paper and tape or string and tape construct a facsimile of these two ligaments on the articulated pelvis. Note that these two ligaments define the boundaries of the greater and lesser sciatic foramina (see Fig. 10 above). Important nerves, blood vessels and muscles pass through these foramina, so you must become familiar with their location and boundaries.

The area lying between the pelvic inlet and the pelvic outlet is considered to be the true Pelvis, or pelvic cavity. The area above the pelvic inlet is sometimes termed the false pelvis, but a quick look at an articulated skeleton or a partially dissected cadaver will show you that this region is really part of the abdominal cavity.

You have now defined the bony and ligamentous framework of the pelvis (above), without consideration of bony differences between males and females. Although many texts delineate these differences ad nauseum, the important differences between male and female peIves are summarized very succinctly in Fig. 11 below.
 

Figure 11. Table summarizing the differences in the shape of the pelvic inlet, pelvic outlet, pelvic cavity and pubic arch in male and female pelves. (From Snell, Clinical Anatomy for Medical Students. Fig. 20 p.075.)
 

Verify these differences in typical shape of male and female bony pelvis by referring to Figs. 3-1 through 3-4 Grant's Atlas. Attempt to obtain both male and female articulated bony pelvis and compare the features highlighted in Fig. 11 above.

Although it is important to understand the architecture of the bony pelvis, you should realize that the walls of the pelvic cavity are formed not only by bones and ligaments, but also by muscles. Two muscles and their fasciae form part of the walls of the pelvic cavity:

  1. The piriformis muscle: Takes origin from the front of the sacrum near the 3rd and 4th sacral foramina. The muscle leaves the pelvis by passing laterally through the greater sciatic foremen and inserts on the upper border of the greater trochanter of the femur. (See Figs. 12 and 13 below, also Atlas Figures 3-12 and 4-36.)
  2. The obturator internus muscle: Takes origin from the inner surface of the obturator membrane, which closes over most of the obturator foremen, and from the interior surfaces of the pubis and ischium, below the Iliopectineal line, and from the pubic ramus. This is a fan-shaped muscle which becomes tendinous as it passes through the lesser sciatic foremen. The tendon inserts on the greater trochanter of the femur. (See Figs. 13 and 14 below, also Atlas Figures 3-12, 3-22, and 4-37.)
  
Fig. 12 (left). Disposition of piriformis (shown) Fig. 13. Dorsal (posterior) view of right side on right side of pelvis) and coccygeus of bony pelvis showing insertions of obturator muscles shown in frontal view of bony internus and piriformis muscles onto greater pelvis.  

Fig. 13 (right). Dorsal (posterior) view of right side of bony pelvis showing insertions of obturator internus and piriformis muscles onto greater trochanter of the femur.
 

Figure 14. Illustrates the origin of the obturator internus muscle. This is a view of the right side of the pelvis, looking at the pelvic walls from the midline. The insertion of the obturator internus tendon on the greater trochanter is illustrated in Fig. 13. Note also that the position of line of attachment of the levator ani muscle and the approximate position of the pudendal canal in the obturator internus fascia are indicated. These will be described in more detail below.
 
Two muscles and their fasciae form the floor of the pelvic cavity, which is also referred to as the pelvic diaphragm:
 
  1. The levator ani muscle: This is a thin sheet of muscle which takes origin from the pubic bone and the adjacent fascia of the obturator internus muscle (see Figs. 15 and 16 below, and Atlas Figure 3-121. Various portions of this muscular sheet insert respectively, on the coccyx; onto a fibrous band running from the rectum and coccyx called the anococcygeal rapine; and onto a fibrous body lying between the prostate or vagina and rectum, known as the perineal body. The levator ani muscle can be subdivided, depending on the exact origin and insertion of the fibers:
    1. levator prostatae or sphincter vaginae - These include the fibers taking origin from the posterior surface of the pubis, and inserting in the perineal body. These form a "sling" around the prostate or vagina. (Not shown in the figures, except Snell, Fig. 15, p. 269.)
    2. puborectalis - These fibers take origin from the posterior surface of the pubis, and form a sling around the rectum. The puborectalis is thought to be important in maintaining fecal continence. (See Atlas Fig. 3-40.)
    3. pubococcvgeus - These fibers take origin from the posterior surface of the pubis and insert into the anococcygeal rapine.
    4. iliococcvgeus - these fibers take origin from the obturator internus fascia and ischium and insert into the anococcygeal rapine.
Try not to become so fixated on defining parts of the levator ani that you forget that the levator ani is basically one muscular sheet.
 
Figure 15. (left) Schematic drawing showing the attachment of the levator ani muscle to the obturator internus fascia. (From Basmajian, Grant's Method of Anatomy, Fig. 17.6, p. 209.)

Figure 16. (right) Drawing summarizing the arrangement of muscles forming the floor and walls of the pelvic cavity. Note that the levator prostatae (sphincter vaginae) portion of the levator ani muscle is not shown. (From: Woodburne, Essentials of Human Anatomy, Fig. 405, p. 512.)

  1. The coccvgeus muscle: This muscle takes origin from the ischial spine and inserts onto the lower part of the sacrum and coccyx. Its disposition is illustrated in Fig. 12. Note that the orientation and shape of the coccygeus muscle are parallel to those of the sacrospinous ligament, but the muscle lies anterior (ventral) to the ligament. (This point is also demonstrated very clearly by comparing Atlas Figures 3-55 and 3-57.)
Figure 16 above summarizes the disposition and relationships of the obturator internus, piriformis, coccygeus and levator ani muscles in forming the muscular floor and walls of the pelvis. (Atlas Figs. 3-12 and 3-13 also serve as good summary figures.

Probably the best way to appreciate the orientation and relationships of the muscles forming the floor and walls of the pelvis is to place paper representations of the muscles in the appropriate positions on an articulated pelvis. To facilitate this exercise, paper templates of these muscles are provided on thelast 2 pages of this guide. Note that for the piriformis, coccygeus, and obturator internus muscles, templates are provided for the muscles on the right of the pelvis only. (To construct muscle models for the left side. make a mirror image copy of the template for the right side.) Muscles will be correctly oriented if you place them in the bony pelvis with the writing facing the interior of the pelvis. Orient the muscles correctly using the descriptions and figures provided above. Hold the "muscles" in place with tape. Note that the obturator internus and piriformis muscles attach to the greater trochanter of the femur in real life. Since you don't have femurs furnished with your articulated pelves, it will be best for you to attach these "muscles" at the acetabulum. The large black dots drawn on the template indicate the approximate position where you should attach the piriformis and obturator internus muscles to the acetabulum to achieve the correct position and orientation. Note that since bony pelves differ somewhat you may have to fidget, a bit to make the paper muscles fit on your bony pelvis.
 
When you feel comfortable with the architecture of the pelvic walls and floor, it is time to move on to a consideration of the regional anatomy of the pelvis. Before considering the anatomy of the pelvis itself, however, it will be appropriate to briefly consider the relationship of the pelvis to the adjoining regions of the body: the abdominal cavity and the lower limb.

It was noted above that the official definition of the pelvis states that the pelvis comprises the area between the pelvic brim (or inlet) and the pelvic outlet. This area can be subdivided into the area above the pelvic diaphragm, termed the pelvic cavity proper; and an area below the pelvic diaphragm, termed the perineum (See Fig. 17).
 

Figure 17. Schematic drawing of the pelvis showing how the pelvic diaphragm subdivides it into the pelvic proper and the perineum. (From: Moore, Clinically Oriented Anatomy, Fig. 3-51, p. 342.)
 

I. Relationship of the pelvis to the abdominal cavity.
 
You should realize from the dissection you have done thus far that the pelvic cavity proper is simply an inferior continuation of the abdominal cavity. In fact, the abdominal and pelvic cavities are sometimes considered as a single cavity (termed the abdominopelvic cavity) with the thoraco-abdominal diaphragm forming the roof and the pelvic cavity forming the floor (Fig. 18). It makes sense, therefore, that certain of the organ systems of the abdominal cavity are continued in the pelvic cavity: the pelvic cavity contains the terminal portions of the gastrointestinal tract (sigmoid colon and rectum) and the urinary system (ureters, bladder, and beginning portion of the urethra). The peritoneum also continues into the pelvis, partially covering the pelvic organs, and forming various ligaments and mesenteries as it drapes over vasculature and other structures passing from the pelvic walls to the viscera. It is important to realize, however, that the peritoneum does not reach all the way to the lowest points of the pelvic diaphragm so that there exists a fairly substantial subperitoneal space containing varius pelvic organs (or portions of pelvic organs), vasculature, and fascial ligaments which play an important role in supporting pelvic viscera.
 

Figure 18. Diagram illustrating the continuity of the abdominal and pelvic cavities. The (thoracoabdominal) diaphragm forms the roof of the abdominopelvic cavity, pelvic diaphragm forms its floor (modified from: Moore' Clinically Oriented Anatomy, Fig. 3-51, p. 342.)  

Figure 19. Diagram illustrating the continuation of the peritoneal cavity into pelvis, and the existence of a substantial subperitoneal space between the peritoneal and the pelvic diaphragm.  

Logically, the major arterial trunks seen in the abdominal cavity will also be continued into the pelvis, while major veins and lymphatic trunks seen in the abdomen will receive blood or lymph from tributaries located in or near the pelvis. Some of the nerves seen in the abdomen also continue into the pelvic cavity. Let us start by considering the arterial blood supply to the pelvis, relating it to what you already know (hopefully) about arteries in the abdomen.
 
The major artery of the abdomen is the descending aorta. It gives off 3 unpaired visceral stalks which supply the gastrointestinal tract (celiac trunk, superior mesenteric artery, and inferior mesenteric artery). These mainly supply abdominal viscera, and Hey the inferior mesenteric artery has any branches which supply the pelvis. The superior rectal artery branches off of the inferior mesenteric, crosses the pelvic brim, and supplies the upper part of the rectum. You may also recall that because the ovaries and testes have an embryonic origin near the kidneys, the arteries which supply these gonadal organs arise from the abdominal aorta. The ovaries and testes take their blood supply with them as they migrate. Since the testes migrate through the inguinal canal to the scrotum, the testicular artery is not seen in the pelvis. However, as the ovaries descend into the pelvis, they drag the ovarian artery with them across the pelvic inlet, and into the pelvis.
 
The abdominal aorta ends by bifurcating at L4, to form right and left common iliac arteries. The common iliac arteries bifurcate again just above the level of the pelvic inlet to form an internal iliac and an external iliac artery. The external iliac artery passes under the inguinal ligament into the thigh; its branches constitute the major arterial blood supply to the lower limb. The internal iliac crosses the pelvic brim to enter the pelvis. The branches of the internal iliac artery constitute the major blood supply to the perineum. The only pelvic organs that don't receive their blood supply from branches of the internal iliac are the ovaries and the superior part of the rectum. (See Figure 20 for a summary of the points made above.)
 

Figure 20. Schematic diagram illustrating the sources of arterial blood supply to pelvic structures.
 
The venous drainage of the pelvis parallels the pattern of the arterial blood supply. Thus, most of the pelvic cavity proper and perineum is drained by tributaries of the internal iliac veins, which drain into the common iliac veins, which drain into the inferior vena cave. The exceptions to this general rule should be obvious if you think about it. First, recall that the structures supplied by branches of the celiac trunk, superior mesenteric artery and inferior mesenteric artery drain into the portal vein. The upper part of the rectum, supplied by the superior rectal branch of the inferior mesenteric artery, will therefore drain into the portal vein by way of the superior rectal and inferior mesenteric veins. Second, the ovaries drain by way of ovarian veins into the inferior vena cave or renal vein.

Lymphatic drainage of a region also generally follows the pattern of venous drainage. Therefore, for the pelvis, most of the pelvic cavity proper drains into nodes located around the internal iliac vessels. These in turn drain into common iliac nodes, which drain into para-aortic nodes. There are exceptions, however. Logically, the lymphatic drainage of the ovaries is to para-aortic nodes in the abdomen, paralleling the blood supply. Less logically, the lymphatic drainage of the perineum is almost entirely to nodes located in the inguinal region. (A summary of lymphatic drainage of the pelvis and perineum is provided in Figure 21 below.)
 

Figure 21. Schematic diagram summarizing the lymphatic drainage of the pelvis end perineum. (From: Rohen and Yokochi, Color Atlas of Anatomy, p. 341.)
 
Most of the nerves seen in the abdominal cavity do not continue into the pelvis. There are exceptions: 1) The obturator nerve. derived from L2-4 crosses over the pelvic brim and travels in the pelvic cavity briefly before exiting through the obturator foremen (Its destination is the adductor compartment of the thigh). 2) There are contributions from L4 and 5 (the Jumbosacral trunk) which cross the pelvic brim and join up with contributions arising from SI-3 to form the sciatic nerve. This nerve leaves the pelvis through the greater sciatic foremen and provides the major innervation to the back of the thigh and muscles of the leg and foot. 3) The sympathetic trunk continues to sacral levels of the pelvis, with postganglionic sympathetic fibers joining sacral and coccygeal nerves. Postganglionic sympathetic nerves also leave the hypogastric plexus (located in front of the sacral promontory) to innervate smooth muscle in pelvic viscera and vasculature.
 
Other nerves found in the pelvis will be essentially native to the pelvis. Recall that the vagus nerve supplies the gut only as far as the left colic flexure. The parasympathetic nerve supply to the pelvis is derived entirely from S2-S4. Most of the somatic sensory and motor supply to the pelvis is also derived from S2-S4.
 
Figure 22. Schematic diagram summarizing the nerve supply to the pelvis. (From: Snell, Clinical Anatomy for Medical Students. Fig. 16, p. 279.)
 

II. Relationship of the pelvis to the lower limb.
 
Considering the close geographical relationship between the pelvis and the proximal part of the lower limb, it is logical that nerves, blood vessels, lymphatics, and muscles passing to and from the limb must either pass through the pelvis, or very near it. Moreover, in addition to providing support and protection for the pelvic organs, the bones of the pelvis also act as a "girdle" or framework for attachment of the bones and muscles supporting the lower limb. The head of each femur articulates with the os coxa at the acetabulum (see Fig. 7).
 
There are 4 major "gateways" to the lower limb. Three of these are located in the walls of the true pelvis; only one bypasses the true pelvis.

  1. Structures passing under the inguinal ligament bypass the true pelvis entirely on their way to the leg. Construct a facsimile of the inguinal ligament on your articulated pelvis by attaching a string to the anterior superior iliac spine and the pubic tubercle. Notice that a considerable space exists between the ligament and the bony pelvis. In this space run several important structures.
    1. The iliopsoas muscle, which inserts on the femur and therefore acts to flex the thigh at the hip joint.
    2. The femoral nerve, which is the major nerve supplying the muscles and skin on the lateral side of the front of the thigh.
    3. The femoral artery and vein, which are the major arterial blood supply and venous drainage for the lower limb. There are direct continuations of the external iliac artery and vein.
    4. L,ymphatics draining into the inguinal nodes.
  2. Structures passing to and from the Tower limb by way of the obturator foremen (see Fig. 15):
    1. The obturator nerve' which supplies skin and muscles on the medial side of the front of the thigh.
    2. The obturator artery and vein, which are branches of the internal iliac artery and vein, and constitute the main vascular supply for the medial side of the front of the thigh.
  3. Structures passing to and from the lower limb by way of the greater sciatic foremen (see Fig. 10):
    1. The piriformis muscle, which inserts on the greater trochanter of the femur (see Fig. 13 above) and acts as an abductor of the thigh at the hip.
    2. The superior glutea] nerves and vessels, which pass superior to the piriformis muscle supply 3 muscles of the gluteal and hip region: gluteus medius, gluteus minimus and tensor fascia lata.
    3. The inferior glutea] nerves and vessels, which pass inferior to the piriformis muscle supply the gluteus maximus muscle.

    4. The sciatic nerve, which is the largest nerve in the body. (It is roughly the thickness of a garden hose!) It exits inferior to the piriformis muscle to the skin and muscle of the back of the thigh, as well as the entire leg and foot (see Fig. 23)
Figure 23. Drawing illustrating the structures entering and leaving the pelvis through the greater sciatic foremen. (Modified slightly from: Basmajian, Grant's Method of Anatomy, Fig. 21.6, p. 258.)
  1. Structures passing to and from the lower limb by way of the lesser sciatic foremen (a minor route, see Fig. 10)
    1. The tendon of the obturator internus muscle, which inserts on the greater trochanter of the femur (see Fig. 13 above) and acts as an abductor of the thigh at the hip. 
Make sure that you can localize each of these gateways to the lower limb on the articulated pelvis. You will dissect the lower limb in detail later in the course. At this point you should mainly be aware as you dissect the pelvis that some of the structures you see in the pelvis are not "native" to the region, but are passing to or from the lower limb.
 

III. Regional anatomy of the pelvic cavity proper

If you have worked through the material above, you Greatly know a great deal about the regional anatomy of the pelvic cavity proper. The remainder of this exercise is intended to help you synthesize what you have learned about pelvic anatomy, and to provide you with a more detailed understanding of the anatomy of the pelvic cavity proper. In terms of contents of the pelvic cavity, we will be concerned mainly with constituents of 3 major organ systems: urinary, gastrointestinal and reproductive. Before we go into details of these systems as they relate to the pelvic cavity proper, it will be useful to recapitulate some general rules as they apply to pelvic anatomy.

  1. The blood supply to the pelvic cavity proper is mainly from branches of the internal iliac artery and vein. Exceptions: ovarian artery' superior rectal artery (see discussion above). (See Fig. 20.)
  2. The lymphatic drainage of the pelvic cavity is mainly to nodes located around the internal iliac vessels. (See Fig. 21.)
  3. The autonomic nerve supply to pelvic viscera is mainly from sympathetic postganglionic fibers emerging from the hypogastric plexus, and parasympathetic preganglionic fibers arising from S2-S4, also called pelvic splanchnics. (Remember, parasympathetic postganglionic neurons tend to be located in the wails of the viscera.) (See Fig. 22.)
  4. Recall that the greater sac of the peritoneal cavity extends part way into the pelvic cavity. Contents ofthe pelvic cavity may therefore be subperitoneal (retroperitoneal) or partly or completely peritonealized.
Let us now consider the parts of the major organ systems located in the pelvic cavity proper:
  1. The urinary system: The pelvic cavity proper contains the terminal portion of the ureters, the bladder, and the beginning of the urethra. The bladder is located behind the pubic symphysis. (See Fig. 24.)
Figure 24. Drawing illustrating a midsagittal section of male and female pelves, showing the relative positions of the pelvic organs and the extent of the peritoneal covering (shaded). (From: O'Rahilly, Basic Human Anatomy, Fig. 3 1-9, p. 28 1.)

When empty, the bladder is entierely shielded by the pubis, but the dome rises above the pubis when the bladder is full. The ureters gain access to the bladder by descending retroperitoneally, crossing the pelvic brim and entering the bladder posterolaterally. (See Fig. 25.) The urethra is located (logically) at the inferior aspect of the bladder. It passes through the prostate gland in the male, while in the female the shorter urethra leaves the pelvic cavity immediately through a hiatus in the pelvic diaphragm. The vascular supply to the bladder is by way of inferior and superior vesicle arteries and veins which are branches of the internal iliac artery and vein. Lymphatic drainage is mainly to internal iliac nodes, but also, to some extent, to external iliac nodes. Sympathetic nerves pass to the bladder, but apparently they only innervate smooth muscle in the walls of blood vessels. Contraction of the smooth muscle in the bladder wall and the involuntary sphincter of the bladder is regulated by parasympathetic nerves (pelvic splanchnics arising from S2-4). Look at Figure 24 and realize that, just as the components of the urinary system are retroperitoneal in the abdominal cavity, they are retro - or subperitoneal in the pelvic cavity. Identify the approximate location of the bladder and urethra in the articulated pelvis.
 

Figure 25. Drawing illustrating the position of the empty bladder behind the pubic symphysis, the fact that the dome of the bladder rises above the pubic symphysis when full, and the entrance of the ureters into the posterolateral angle of the bladder. Lateral view of the left side of the bladder. (From: Snell, Clinical Anatomy for Medical Students, Fig. Sa, p. 289.)

The gastrointestinal system: The pelvic cavity proper contains part of the sigmoid colon and the rectum, mainly the rectum. The rectum is located posteriorly in the pelvic cavity, in front of the body of the sacrum. Just as the upper parts of the gastrointestinal tract have a varying relationship to the peritoneum, the pelvic portions of the gastrointestinal tract vary in their relationship to the peritoneum. The sigmoid colon has a mesentery and is intraperitoneal. The upper part of the rectum is covered anteriorly and laterally by peritoneum, while the middle part is covered only anteriorly, and the lowest part has no peritoneal covering (see Fig. 24). The upper part of the rectum receives its blood supply from the superior rectal artery, a branch of the inferior mesenteric. The lower 2/3 of the rectum is supplied by branches of the internal iliac: the middle third by the middle rectal artery; the lower third by the inferior rectal arteries which branch off of the internal pudendal artery in the perineum (see Fig. As).
 

Figure 26. Drawing illustrating the blood supply to the rectum. (From: Basmajian, Grant's Method of Anatomy, Fig. 17.33, p. 222.)

Venous drainage parallels the arterial blood supply. The lymphatic drainage of the pelvic rectum is mainly to internal iliac nodes. Sympathetic (from the hypogastric plexus) and parasympathetic (pelvic splanchnic) nerves innervate the rectum.

  1. The reproductive system: male and female internal reproductive organs are housed in the pelvic cavity. In the male the pelvic components of the reproductive system are located behind and beneath the bladder.
In the female they are located between the bladder and rectum (see Fig. 24). Before considering the detailed anatomy of the pelvic reproductive organs, it will be helpful to review the overall organization of the male and female reproductive systems. The easiest way to do this will be to trace the path of a spermatozoa through the male reproductive system and the path of an egg through the female reproductive system. 
 
Figure 27. Drawing summarizing the male reproductive system. (From: Hole, Human Anatomy and Physiology, Fig. 21.1, p. 714.)
Figure 28. Drawing summarizing the anatomy of the female reproductive system. (From: Tortora and Anagnostakos, Prinoiples of Human Anatomy And Physiology, Fig. 28-10, p. 720.)
Important information shout male pelvic reproductive system.
  1. The vas deferens pass behind the prostate gland.
  2. The seminal vesicles also lie posterior to the bladder, lateral to the vas deferens. (See Fig. 29 below.)
  3. The ducts of the seminal vesicles join up with the ductus deferens to form the ejaculatory ducts in the substance of the prostate gland.
  4. The ejaculatory ducts empty into the prostatic urethra, and from that point on spermatozoa travel through the urethra to get to the outside (or inside) world.
  5. Secretions of 3 pairs of glands, the seminal vesicles, prostate and bulbourethral glands are added to the substance that is ejaculated (semen). Secretions of the seminal vesicles and prostate seem to play an important role in sperm motility. Secretions of the bulbourethral glands appear to be more important for lubrication during sexual arousal.

  6.  
Important information about female reproductive system
  1. Although the ovaries are completely surrounded by peritoneum (they are intraperitoneal and have a short mesentery, the mesovarium) when the ovarian follicle ruptures, the ovum ruptures through the peritoneal covering into the peritoneal cavity. The ovum is then picked up by the fimbriae at the expanded end of the uterine tube, called the infundibulum, which are highly motile. An ovum produced by the left ovary may be picked up by, and migrate through either the right or left uterine tube. The uterine tubes open into the perito neat cavity. Therefore in the female, the potential exists for infection to spread from the uterine tubes to the peritoneal cavity.

  2.  
Figure 29. Drawing illustrating a posterior view of the bladder, ductus deferens, seminal vesicles and prostate in the male. (From: Tortora and Anagnostakos, Principles of Human Anatomy and Physiology, Fig. 28-7, p. 717.)
 

More about the regional anatomy of the pelvis as related to the reproductive system...
 

Blood supply to the intenal reproductive organs.
 
Male: The prostate receives branches from the inferior vesicle and middle recta] arteries. The venous drainage is noteworthy: the prostatic plexes can drain toward either the vertebral venous plexus or toward internal iliac veins. The former route provides a pathway for spread of prostatic cancer to the spine. (See Fig. 30 below for a summary of the branches of the internal iliac artery in the male pelvis.)
 

Figure 30. Summary of the branches of the internal iliac artery in the male pelvis. (From: Basmajian, Grant's Method of Anatomy, Fig. 17.35, p.233.)

Female: The ovaries are supplied by the ovarian arteries (branches of the abdominal aorta). The uterus is supplied by the uterine artery, which branches off of the iliac artery (see Fig. 31, note the major anastomotic connection between ovarian and uterine arteries).
 
Figure 31. Drawing summarizing the blood supply to the female internal reproductive organs. (From: Basmajian, Grant's Method of Anatomy, Fig. 18.10, p. 233.)

The anatomic relationship between the ureter and uterine artery is worthy of note. The ureter passes inferior to the uterine artery near the lateral fornix of the vagina. Care must be taken not to clamp the ureter when tying off the uterine artery during hysterectomy. The blood supply to the upper part of the vagina is from vaginal branches of the uterine artery and the vaginal artery, a branch of the internal iliac artery.

Lymphatic drainage of the internal reproductive organs.

Male: The prostate drains toward internal iliac nodes.

Female: Most of the uterus and upper part of the vagina drain toward internal iliac nodes. The fundus of the uterus is said to drain along the round ligament toward inguinal nodes. The ovaries drain to paraaortic nodes.

Nerve supply to the internal reproductive organs: The internal reproductive organs in male and female receive parasympathetic innervation by way of pelvic splanchnics and sympathetic innervation by way of branches of the hypogastric plexus.

Supports of the pelvic viscera: There are two major supports for the pelvic viscera in male and female: the pelvic diaphragm, and the subperitoneal fascia which lies between the pelvic diaphragm and the lower recesses of the peritoneal sac. In the male this fascia is condensed to form puboprostatic ligaments which help to support the prostate. In the female this fascia is condensed to form 3 pairs of ligaments which run between the bony pelvis and the cervix (and upper part of the vagina. These are known as pubocervical, transverse cervical, and sacrocervical ligaments (see Fig. 32 below).
 

Figure 32. Drawing showing the relative positions of the fascial ligamentous supports of the pelvic viscera in the female. A. As seen from below. B. A modified and highly schematic parasagittal section through the female pelvis. (Modified from: Snell, Clinical Anatomy for Medical Students, Fig. 14, p. 302.)

The broad ligament in the female (there is no broad ligament in the male!) is formed by the draping of peritoneum over the uterus, uterine tubes, ovaries, and blood vessels, nerves, lymphatics and ligaments passing and from these structures. Realize that the broad ligament and its specializations do not fulfill much of a supportive function in the female. (This role is carried out mainly by the pelvic diaphragm and subperitoneal fascia.)  

The broad ligament can be subdivided into several specialized areas and smaller ligaments. If you consider the orientation of the female reproductive organs and associated structures beneath the peritoneal draping, the specialized folds and ligaments visible within the broad ligament are logical. The following structures are contained within the broad ligament (see Figures 33 and 34):

  1. The uterine tubes: These generally extend out posterolaterally from the superior part of the uterus. The broad ligament drapes over them.
  2. The ovaries: These are generally located in the posterior part of the broad ligament. This means that they are visible (and palpable) through the posterior sheet of peritoneum making up the broad ligament.
  3. Blood vessels passing to and from the uterus (uterine arteries), ovaries (ovarian arteries) and uterine tube (anastomotic connections between ovarian and uterine arteries, see Fig. 31 ): The area of the broad ligament near the uterus where the uterine artery passes between two sheets of peritoneum forms a mesentery for the uterus called the mesometrium. Similarly, the short mesenteries containing the blood supply to the uterine tube and ovary are known as the mesosalpinx and mesovarium, respectively. The fold of peritoneum which drapes over the ovarian vessels as they pass from the pelvic wall to the ovary is known as the suspensory ligament of the ovary.
  4. Remnants of the gubernaculum: Recall that in the male the gubernaculum is anchored to the inferior part of the scrotum and serves to draw the testes from the abdomen, through the inguinal canal, and into the scrotum during fetal development. A similar process occurs in the female, but in the female the round ligament is anchored in the labia majora, and the ovaries descend only to the pelvic cavity. The ovaries do not pass through the inguinal canal with the gubernaculum. When the ovaries have descended to the pelvis, part of the gubernaculum adheres to the uterus, near the opening of the uterine tube. This remnant of the gubernaculum, which runs from the ovary to the uterus becomes the round ligament of the ovary (or simply the ligament of the ovary). The remainder of the gubernaculum, running from the uterus through the inguinal canal to the labia majora, becomes the vestigial structure known as the round ligament of the uterus.
 
Figure 33. Drawing illustrating the female pelvic cavity as seen from above, with the major components of the broad ligament. (From: Tortora and Anagnostakos, Principles of Human Anatomy and Physiology, Fig. 28-11, p. 721.)
 
Figure 34. Two schematic drawings illustrating the parts of the broad ligament and underlying structures. A. Anterior view, showing the derivatives of the gubernaculum ovarii on the left and the vasculature to the ovary, uterine tube and uterus on the right. B. A parasagittal section through the broad ligament (near the lateral border of the uterus) illustrating its major components. (Modified from: Basmajian, Grant's Method of Anatomy, Fig. 18.5, p. 229.)
 
Regional anatomy of the perineum
 
Look at an articulated pelvis with paper muscles in place. Recall that the perineum is the area below the pelvic diaphragm and is therefore best viewed inferiorly (see Figs. 1 7 and 35). The superior boundary of the perineum is formed by the pelvic diaphragm. The walls of the perineum are formed by the obturator internus muscle, and by the bones and ligaments defining the pelvic outlet (see Fig. 8). Verify this on the articulated pelvis. The inferior boundary of the perineum is formed by the skin covering this region. Seen from below, the perineum is a diamond-shaped area, which can be subdivided into an anterior part, called the urogenital triangle, and a posterior part, called the anal triangle (see Fig. 35).  
Figure 35. Inferior view of the pelvic outlet, showing the bony and ligamentous boundaries of the perineum and its subdivision into anal and urogenital triangles. (From: Woodburne, Essentials of Human Anatomy, Fig. 375, p. 465.)
 
Figures 36 and 37 show the externally visible structures of the perineum in the male and female, with the boundaries of the anal and urogenital triangles super imposed on the drawings. Realize that the major structures of importance in the anal triangle will be the anal canal, its external sphincters (not visible), and nerves, blood vessels and lymphatics passing to or from the anal canal.

The important contents of the urogenital triangle in both sexes will include the external genitalia and underlying erectile tissue, the lower part of the urethra, a musculofascial diaphragm called the urogenital diaphragm (part of which acts as a voluntary sphincter for the urethra), glands which produce a lubricating substance (bulbourethral glands in the male, greater vestibular glands in the female), and nerves, blood vessels, and lymphatics passing to or from these structures. In the female, the lower part of the vagina is also situated in the urogenital triangle.
 

Figure 36. Inferior view showing the externally visible structures of the male perineal region and its subdivision into anal and urogenital triangles.
 
Figure 37. Inferior view showing the externally visible structures of the female perineal region and its subdivision into anal and urogenital triangles. (Modified from: Snell, Clinical Anatomy for Medical Students, Fig. 37, p. 321.)
 
Before considering the regional anatomy of the perineum in more detail, it will be useful to make some general points concerning the anatomy of the perineum.
 
1.    The somatic nerve supply (motor and sensory) to this region comes almost entirely from branches of the pudendal nerve. This nerve arises from S2-4 in the pelvis, leaves the pelvic cavity proper by way of the greater sciatic foremen, and enters the perineum below the pelvic diaphragm by way of the lesser sciatic foremen. In the perineum, the pudendal nerve travels anteriorly in a fascial tunnel on the surface of the obturator internus, called the pudendal canal. Verify this by recreating the course of the pudendal nerve on your articulated pelvis using string (see Fig. 38).
 
Exceptions to rule 1: The genitofemoral nerve (Ll and 2) supplies motor innervation to the cremaster muscle and sensory innervation of a posterolateral area of the scrotum in the male. It mediates sensory innervation of a small posterolateral area of the labia majora in the female. The ilioinguinal nerve (Ll) mediates sensory innervation of a small anterolateral region of the scrotum or labia majora.

2.    There is also parasympathetic and sympathetic innervation passing to vasculature, erectile tissue, and glands in this region. The parasympathetic and sympathetic nerves travel with blood vessels and branches of the pudendal nerve supplying the perineum.
 

Figure 38. Schematic drawing showing the course of the pudendal nerve in the male (pseudoparasagittal view). The trajectory of the nerve is similar in the female, except that branches pass to the labia majora and clitoris, rather than scrotum and penis. (Modified from: Snell, Clinical Anatomy for Medical Students, Fig. 6, p. 326.)
 
3.    The major arterial blood supply and venous drainage of the perineum is by way of branches of the internal pudendal artery and vein. The internal pudendal artery and vein follow a trajectory similar to that described for the pudendal nerve (see Fig. 38 above).
 
Exception to rule 3: The anteromedial regions of the scrotum and labia majora receive some of their vascular supply from deep and superficial external pudendal arteries, which are branches of the femoral artery.
 
4     The lymphatic drainage of the perineum is mainly to superficial inguinal nodes.
 
5.    The urogenital triangle can be further subdivided into inferior and superior portions by the existence of a musculofascial diaphragm called the urogenital diaphragm. This bears some explaining: Think of the pelvic cavity as a two story building. The upper story is the pelvic cavity proper, the lower story is the perineum.
  
The pelvic diaphragm, which forms the floor of the pelvic cavity proper, therefore forms the ceiling or roof of the perineum. The perineum can be subdivided into an anal triangle and a urogenital triangle, as describer} above. Imagine now that the schematic drawing below is a view of the pelvis from the side:
The urogenital diaphragm is like a mezzanine or balcony in the first story of the two-story building, jutting out into the space of the urogenital triangle, but not into the anal triangle. The urogenital diaphragm fills in the area between the pubic rams, dividing the urogenital triangle into a part inferior to the diaphragm, called the superficial pouch, a space between the fascial sheets forming the diaphragm, called the deep pouch, and a space between the pelvic diaphragm and the urogenital diaphragm, which is not usually given a special name.
The urogenital diaphragm consists of 3 layers: an inferior layer of fascia, known as the perineal membrane; a muscle laver, containing the sphincter urethrae (which acts as a voluntary sphincter for the urethra) and deep transverse perineal muscles; and a superior layer of fascia. The muscle layer is "sandwiched" between the two fascial sheets, which fuse anteriorly and posteriorly. (See Fig. 43 below for a further explanation of the urogenital diaphragm.) Posteriorly, the fascial sheets and the insertions of the deep transverse perineal muscles fuse at the perineal body. As pointed out above, the space between the inferior and superior fascia of the urogenital diaphragm comprises the deep perineal pouch.
 

The anal triangle
 
As stated above, the major structure of the anal triangle is the anal canal. The anal canal contains an internal sphincter of smooth muscle within its walls, which is involuntary. Around the anal canal, on its external surface, passes the external anal sphincter, which is a striated muscle, and therefore under voluntary control. The disposition of the subcutaneous, superficial and deep portions of the external anal sphincter is summarized in Fig. 39. Note that the puborectalis portion of the levator ani forms a "sling" around the rectum, which helps to maintain fecal continence.
 

Figure 39. Drawing showing the arrangement of the puborectalis muscle, and the deep, superficial, and subcutaneous portions of the external anal sphincter. (From: Snell, Clinical Anatomy for Medical Students. Fig. 7, p. 324.)
 
Figure 40. Diagram illustrating the puborectalis sling, which forms an angle in the rectum between the anal canal and rectum, helping to maintain fecal continence. (From: Grant's Atlas, Fig. 3-40.)

The nerve supply to the external anal sphincter is by way of inferior rectal nerves, which are branches of the pudendal nerve (see Fig. 38). The blood supply to the upper part of the anal canal is from branches of the superior rectal artery, which continue from the pelvic cavity. Lymphatic and venous drainage parallel the arterial blood supply. The blood supply to the lower part of the anal canal is from inferior rectal arteries and veins, which are branches of the internal pudendal artery and vein. Lymphatic drainage is to the superficial inguinal nodes.

There is a wedge-shaped space on either side of the anal canal which is bounded superomedially by the pelvic diaphragm, laterally by the obturator internus, and inferiorly by the skin. This space is known as the ischiorectal fossa. In life this space is filled with fat, which is almost liquid at body temperature and allows for easy distension of the anal canal during defecation (see Fig. 41).
 

Figure 41. Frontal section through the ischiorectal fossa, showing the disposition of the anal canal, external anal sphincter, and the boundaries of the ischiorectal fossa. (From: Basmajian, Grant's Method of Anatomy, Fig. 16.5, p. 199.)
 

The urogenital triangle
 
The externally visible structures of the urogenital triangle have been illustrated in Figs. 36 and 37. In the male these are the penis and scrotum; in the female these include the labia majora, labia minora, clitoris, and urethral and vaginal orifices. Immediately beneath the skin of the urogenital triangle is a layer of membranous fascia, which is continuous with the superficial membranous fascia of the abdomen. The membranous fascia fuses with the superficial fascia of the urogenital diaphragm (perineal membrane) at its posterior border.
 
Deep to the superficial membranous fascia, and lying between it and the perineal membrane, is the superficial pouch. The major contents of the superficial pouch are columns of erectile tissue and thin sheets of muscle covering the erectile tissue and the posterior portion of the perineal membrane, nerves supplying these muscles and the overlying skin (perineal branches of the pudendal nerve), and blood vessels supplying the region (branches of internal pudendal vessels). Figure 42 shows the disposition of the erectile tissue in the superficial pouch in males and females.
 

Figure 42. Columns of erectile tissue forming the root and body of the penis in the male, and comprising the crura of the clitoris and bulb of the vestibule in the female. (From: O'Rahilly, Basic Human Anatomy, Fig. 38-4, p.313.)  

In the male, three columns of erectile tissue comprise the root of the penis: the two crura and the bulb of the penis. The anterior continuations of these columns of erectile tissue, the (2) corpora cavernosa and the corpus spongiosum, fuse to form the body of the penis. The expanded continuation of the corpus spongiosum forms the glans of the penis. Realize that the penile portion of the urethra runs through the corpus spongiosum to its orifice in the glans.

In the female, there are four columns of erectile tissue in the superficial pouch: two crura of the clitoris, which fuse anteriorly to form the clitoris; and the two bulbs of the vestibule.

The columns of erectile tissue in both male and female are covered over by thin sheets of muscle. The crura of the clitoris and penis are covered by the ischiocavernosus muscle; while the bulb of the penis and bulbs of the vestibule are covered by the bulbospongiosus muscle. A thin strip of muscle also covers the posterior portion of the perineal membrane: this is called the superficial transverse perineal muscle (summarized in Fig. 43 e and f).

In the female, the greater vestibular glands also sit in the superficial pouch just posterior to the bulbs of the vestibule. Their ducts open onto the mucous membrane between the hymen and the labia minora. The fluid secreted by these glands performs a lubricating function during sexual arousal.

The urogenital diaphragm has been described above. Figure 43 b-d illustrates the layers of the urogenital diaphragm, and the disposition of the sphincter urethrae and deep transverse perineal muscles in the deep perineal pouch. Realize that the urethra passes through the urogenital diaphragm in both male and female. In the male, the portion of the urethra located within the deep perineal pouch is known as the membranous urethra.
 
The contents of the deep perineal pouch in the male also include the bulbourethral glands which produce a lubricating fluid. Note, however, that the ducts of these glands open into the urethra in the superficial pouch. just below the perineal membrane.  

The vagina also passes through the deep perineal pouch in the female.  

In both sexes the contents of the deep pouch also include blood vessels and nerves supplying this region. These are branches of the pudendal nerve and internal pudendal vessels.
 

Figure 43. Schematic diagrams summarizing the contents of the urogenital triangle in male and female. Note that level F is the most superficial, A is the deepest. (From: Moore, Clinically Oriented Anatomy, Fig. 3-16, p. 309.)
 

Part 2. Directions for carrying out the dissection of pelvis and perineum.  

Begin by tracing the inferior continuation of the peritoneal sac from the abdomen into the pelvis.

In the male, note the relationship of the peritoneum to the bladder, ureters, and rectum (see Fig. 24). Locate the rectovesicle pouch between the bladder and the rectum. This is the lowest portion of the peritoneal cavity in the male (in the upright position). Gently strip off the peritoneum to expose the underlying structures.

If you have enough visibility, identify the bladder, rectum, prostate, and puboprostatic ligaments at this time. Sever the puboprostatic ligaments on the right side. Attempt to identify the muscles forming the floor and walls of the pelvic cavity proper (Atlas Fig. 3-12). Sever the attachment of the levator ani muscle to the obturator internus muscle on the right side only (see Fig. 15).
 
In the female, note the relation ship of the peritoneum to the bladder, uterus, and rectum (see Fig. 24). Locate the vesicouterine pouch between the bladder and the uterus, and the rectouterine pouch between the uterus and the rectum. The rectouterine pouch is the lowest portion of the peritoneal cavity in the female (in the upright position). Identify the components of the broad ligament (see Figs. 33 and 34). Attempt to leave the broad ligament intact on the right side, but strip off the remainder of the peritoneum to expose the underlying structures.

If you have enough visibility, identify the bladder, rectum, uterus, uterine tubes, and ovary at this time. Try to identify the pubocervical, transverse cervical, and sacrocervical ligaments (see Fig. 32). Sever these ligaments on the right side. Attempt to identify the muscles forming the floor and walls of the pelvic cavity proper (Atlas Fig. 3-13). Sever the attachment of the levator ani muscle to the obturator internus fascia on the right side only (Fig. 15).

You will have much better access to the structures of the pelvic cavity proper and perineum if you can widen the pelvic inlet and spread the legs of the cadaver. This is accomplished by "disarticulating" the pelvis. The next few paragraphs and figures will tell you how to accomplish this operation.

1.    If this hasn't already been done, remove the skin from the front of the symphysis pubis. Do not skin the external genitalia at this time. Palpate and sever the suspensory ligament of the penis or clitoris. By blunt dissection, using the handle of your scalpel or forceps, detach all of the soft tissue structures from the inferior border of the symphysis pubis. With a scalpel blade, cut through the symphysis pubis, avoiding the urinary bladder. BE CAREFUL that the blade does not pop off of your scalpel handle during this operation and cut you. Clean the front of the pubic bone on the right side to about 4 cm. from the midline. With a saw, make a cut through the pubis roughly 3 cm. from the midline and remove the wedge of bone (see Fig. 44 below). Again, take care not to cut into the soft tissues of the pelvic cavity.
 

Figure 44. Drawing indicating the approximate position of cuts to be made through the symphysis pubis and pubic bone.
 
2.    Turn the cadaver to the prone (face down) position. Make skin incisions as shown below to expose the gluteal region (see Fig. 45). Remove skin and superficial fat in the region of the buttocks, skin only over the posterior thigh.
 
Figure 45. Diagram indicating the skin incisions which should be made to expose the gluteal region. (From: Grant's Dissector, Fig. 3.15, p. 56.)  

This will expose the gluteus maximus muscles (see Fig. 46).
 

Figure 46. Diagram the disposition of the gluteus maximus muscle on the Eight side. (From: Grant's Atlas, Fig. 4-35.)  

Clean and expose the superior, medial, and inferior borders of the gluteus maximus on both sides. On both sides. detach the upper medial border from the ilium. Then detach the medial portion from the sacrum and coccyx. Palpate the medial part of the inferior border of the gluteus maximus. Note that it is attached to the sacrotuberous ligament. Carefully detach the gluteus maximus from the sacrotuberous ligament. Do not cut the sacrotuberous ligament at this time. Reflect the gluteus maximus muscle laterally to the point where the inferior gluteal vessels enter it. Try not to sever the inferior gluteal nerves and vessels. Remember, the goal at this moment is not to do a complete dissection of the gluteal region (this will be done in a later dissection). Your goal here is to gain sufficient exposure of structures of the pelvis and perineum and to be able to disarticulate the pelvis.

Identify the sacrospinous and sacrotuberous ligaments and the greater and lesser sciatic foramina on both sides. Locate the internal pudendal vessels and pudendal nerve as they leave the pelvis through the greater sciatic foremen, loop around the sacrospinous ligament, and enter the perineum through the lesser sciatic foremen (see Fig. 23).

ON THE RIGHT SIDE ONLY, cut the sacrotuberous ligament close to the ischial tuberosity; cut the sacrospinous ligament close to the ischial spine (see Fig. 47 below). DO NOT CUT THE PUDENDAL NERVE AND VESSELS
 

Figure 47. Draining showing where the sacrotuberous and sacrospinous ligaments should be cut.
 
3.    Return the cadaver to the supine position. Place a block under the small of the back. Check to see that the psoas muscle has been removed on the right side in the course of the previous dissection. (If the right psoas has not been removed, check with your instructor as to the best way to accomplish this.) You are non ready to disarticulate the pelvis. Your goal is to break apart the right sacroiliac joint by abducting the lower limbs. Force the feet of the cadaver up to 1 meter apart. As you do this you should hear the noise caused by the tearing of the sacroiliac ligaments. Keep the lower limbs abducted using a wooden block between the legs, or use ropes to tie the limbs in an abducted position. Palpate the region of the right sacroiliac joint to see if there a gap there. If you have been unable to disarticulate the joint, seek the assistance of an instructor.
 
Now that you have widened the access to the pelvis, complete your exploration of the pelvic cavity proper Trace the elements of the urinary and gastrointestinal systems from the abdomen to the pelvic cavity. Trace the course of the male or female reproductive tracts (see Figs. 27 and 28). Identify the vasculature supplying these organ systems. Identify the fascial supports of the prostate and the uterus (Fig. 32). Identify the muscles forming the walls end floor of the pelvic cavity (Figs. 12-16,Atlas Figs. 3- 1 2, 3- 13). Trace the sources of arterial blood supply to the pelvic cavity, including the major branches of the internal iliac artery (see Figs. 20 and 30). Follow the sympathetic trunk from the abdomen to sacral levels. Trace some of the branches of the hypogastric plexus if they are readily visible. Do not spend time on this if the hypogastric plexus is not readily visible (Fig. 22). Trace the obturator nerve from its origin at L2-4 in the abdomen into the pelvis and through the obturator foremen (see Fig. 15). Locate the contributions from L4 and 5 which form the lumbosacral trunk, crossing the pelvic brim to join with contributions from S 1-3 to form the sciatic nerve. Trace the sciatic nerve to where it exits the pelvis through the greater sciatic foremen. Note the formation of the pudendal nerve from S2-4. Trace this nerve as it leaves the pelvic cavity through the greater sciatic foremen together with the internal pudendal vessels (see Fig. 22).
 
Now turn your attention to dissection of the perineum:
 
In the male, remove the skin from the scrotum, penis, and anal region out to the boundaries of the pelvic outlet.
 
In the female, skin the entire perineum except for the mucous membrane enclosed by the labia Sonora. Cut the edges of the labia minora so that you have roughly 1/4 inch of labia minora left, forming a rim around the mucous membrane.
 
When you have finished skinning, turn your attention to the anal triangle. This can be dissected with the cadaver in either the prone or supine position, whichever you find more convenient. By blunt dissection, shell the fat out of the ischiorectal fossa. Your goal is to expose the inferior rectal nerves and vessels passing to the anal canal (see atlas Fig. 3-17). Trace these nerves beck to their origin from the pudendal nerve and internal pudendal vessels. Trace pudendal nerves and vessels through the pudendal canal. in the wall of the ischiorectal fossa (see Fig. 38). Clean and identify the external anal sphincters.

Now turn your attention to the urogenital triangle. Clean any remaining fat from the region. Using Fig. 43 as a guide, attempt to identify the structures of the male and female urogenital triangle. Remember, the muscles of this region are usually very thin, so dissect cautiously. Erectile tissue has a characteristic reddish spongy appearance and should be easy to identify.

If you have a male cadaver, dissect the penis to identify the paired dorsal arteries and nerves, which are branches of the internal pudendal artery and pudendal nerve, respectively. Identify the single deep dorsal vein of the penis and attempt to trace it back to where it drains into the prostatic venous plexus. Verify that the paired corpora cavernosa pass on the dorsum of the penis (nearest the abdomen), while the corpus spongiosum passes on the ventral side of the penis (nearest the lower limb). Verify that the urethra passes through the corpus spongiosum and opens to the outside world through an orifice in the glans, which is the expanded end of the corpus spongiosum. Use Fig. 48 as a guide for your dissection. It is not necessary to transect the penis, however.
 

Figure 48. Dissection of the penis. (From: Grant's Dissector, Fig. 3.19, p. 59.)

When you have completed the dissection, review your work by running through the anatomy, function, and nerve and vascular supply of the reproductive system and the pelvic portions of the gastointestinal and urinary systems.