In this block, we will consider two regions of the body, the abdomen and pelvis, and the anatomy of the internal organs found there. Although often considered separately, the abdomen and pelvis. They act together to provide multiple vital functions including: support and protection of the digestive and urinary tracts and internal reproductive organs and their associated neurovascular supplies; transmission of the neurovascular supply to and from the thorax and the lower limb; provision of support and attachment to the external genitalia and access to and from the internal reproductive and urinary organs; provision of accessory muscles of physiological actions such as respiration, defecation, and micturition; support for the spinal column in weight bearing and movement.
OSTEOLOGY and MUSCULOSKELETAL BOUNDARIES OF THE LESSER PELVIS
Osteology
Refer to an articulated bony pelvis and a skeleton. The pelvis (L. pelvis, basin) is formed by two hip bones (coxal bones, ossa coxae) joined posteriorly by the sacrum. Each hip bone is formed by three fused bones:pubis, ischium, and the ilium. These three bones are fused at the acetabulum. The coccyx is attached to the sacrum. As you study isolated bones such as the hip bone, compare its features with the articulated bony pelvis and the skeleton.
- Pubic symphysis - midline fusion between the bodies of the right and left pubic bones
- Pubic crest - superior margin of the body of the pubic bone directly lateral to the pubic symphysis. The pubic crest lies within the coronal plane.
- Pubic tubercle - prominence located at the lateral end of the pubic crest
- Superior pubic ramus - stout process extending posterolaterally from the pubic tubercle to the iliopubic eminence (see below). The superior pubic ramus forms the superior boundary of the obturator foramen.
- Iliopubic eminence - prominence at a point of fusion between the ilium and the superior pubic ramus; medial to the AIIS is a shallow groove, over which the psoas major and the iliacus muscles pass on their way to the lower extremity; this groove is bounded medially by the iliopubic eminence.
- Ischiopubic ramus - formed by the ischial ramus, a process that extends superomedially from the body of the ischium, and the inferior pubic ramus, a process extending inferolaterally from the body of the pubic bone. These two rami meet roughly halfway to form a structure collectively called the ischiopubic ramus. The ischiopubic ramus forms the inferior boundary of the obturator foramen.
- Pubic arch - bony arch formed by the right and left ischiopubic rami, right and left bodies of the pubic bones, and the inferior end of the pubic symphysis. Note that the subpubic angle (angle of the pubic arch) is wider in females than in males.
- Ischial tuberosity - prominence at the lateral end of the ischiopubic ramus. The ischial tuberosity is the bony structure that bears one’s weight when one sits.
- Ischial spine - pointed process extending posteriorly from the body of the ischium.
- Sacral promontory - most anteriorly projecting part of the S1 vertebra.
- Anterior sacral foramina - openings permitting the exit of the ventral rami of sacral spinal nerves.
3. Identify the coccyx at the caudal end of the sacrum.
4. Examine the ischium. Note that the ischial spine divides the posterior margin of the body of the ischium into two large notches - the greater sciatic notch (located superior to the ischial spine) and the lesser sciatic notch (inferior to the ischial spine).
5. Using an atlas illustration, study the joint between the sacrum and ilium. The sacroiliac articulation is a synovial joint between the auricular surfaces of the sacrum and the ilium. The sacroiliac articulation is strengthened by an anterior sacroiliac ligament and a posterior sacroiliac ligament. The articulation between the ilium and the L5 vertebra is strengthened by the iliolumbar ligament. Do not attempt to find these ligaments on the skeleton or cadaver.
6. The hip bone and sacrum are connected by strong ligaments. On a model with pelvic ligaments, identify thes acrotuberous ligament, a ligament extending from the inferolateral angle of the sacrum (“sacro-”) to the ischial tuberosity (“-tuberous”).
7. Identify the sacrospinous ligament, a ligament extending from the inferolateral angle of the sacrum (“sacro-”) to the ischial spine (“-spinous”).
8. Note that the sacrotuberous and sacrospinous ligaments convert the greater and lesser sciatic notches into greater and lesser sciatic foramina, respectively. The greater sciatic foramen is located superior to the sacrospinous ligament. The lesser sciatic foramen is located inferior to the sacrospinous ligament, between it and the sacrotuberous ligament.
9. Identify the pelvic inlet (superior pelvic aperture), a large communication between the greater pelvis (also called the false pelvis or pelvis major) and the lesser pelvis (also called the true pelvis, pelvis minor, or the “obstetric pelvis”).
10. The bony ridge forming the border around the pelvic inlet is called thepelvic brim (lower right panel). From anterior to posterior, identify the structures that form the pelvic brim:
- Superior end of the pubic symphysis
- Posterior border of the pubic crest
- Pecten pubis (pectineal line) - a sharp line passing along the back edge of the superior pubic ramus. NOTE: The pectineal ligament (of Cooper) is an extension of the lacunar ligament (the lacunar ligament is part of the aponeurosis of the external oblique muscle that is reflected backward and laterally to attach to the pectineal line; recall that it is also rigid medial boundary of the femoral ring) along the pectineal line of the superior pubic ramus. Cooper’s ligament is used in surgical procedures to support pelvic visceral structures that have prolapsed (e.g. the most successful type of pelvic organ prolapse surgery is retropubic suspension. This procedure involves the attachment of the prolapsed structure to Cooper’s ligament). You will hear a lot about this in the OB/GYN clerkship.
- Arcuate line of the ilium - continuation of the pectineal line between the ilium and the ischium toward the sacrum.
- Anterior border of the ala (wing) of the sacrum
- Sacral promontory
Musculoskeletal Boundaries of the Lesser Pelvis
Inguinal Canal
The scrotum is an outpouching of the anterior abdominal wall, and most layers of the abdominal wall are represented in its structure. The superficial fascia of the scrotum contains no fat. Instead, the superficial fascia is represented by dartos fascia (continuous with Colles fascia of the perineum and Scarpa’s fascia, the membranous layer of the superficial fascia of the anterior abdominal wall), which contains smooth muscle fibers (dartos muscle). The scrotum should be opened by a vertical incision along its anterior surface. Observe that the scrotal septum divides the scrotum into left and right compartments. The spermatic cord will be followed from the superficial inguinal ring into the scrotum.
Dissection Instructions: Spermatic Cord
The spermatic cord contains the ductus deferens, testicular vessels, lymphatics, and nerves. The contents of the spermatic cord are surrounded by three fascial layers, the coverings of the spermatic cord, which are derived from layers of the anterior abdominal wall. These coverings are added to the spermatic cord as it passes through the inguinal canal.
ISCHIOANAL FOSSA
Dissection Overview: Gluteal Region
You studied the gluteal region as part of the Musculoskeletal-Integumentary Block. You will return to the gluteal region to trace the pudendal nerve and the internal pudendal artery where they pass through the greater sciatic foramen inferior to the piriformis muscle, and then pass through the lesser sciatic foramen between the sacrotuberous and sacrospinous ligaments. From there they pass into the anal triangle.
The perineum is a diamond-shaped area between the upper thighs and between the lower parts of the buttocks. It consists of structures that constitute the region below the pelvic floor. The perineum is bounded by the pubic symphysis anteriorly, the coccyx posteriorly, and the two ischial tuberosities laterally. Between the pubic symphysis and ischial tuberosities are the rami of the pubic bone and the rami of the ischia (since it is difficult to demarcate where the inferior ramus of the pubic bone ends and the ischial ramus begins, it is common parlance to call these structures “ischiopubic rami”). Extending from the ischial tuberosities to the coccyx (and sacrum) are the sacrotuberous ligaments. For descriptive purposes, it is convenient to divide the diamond-shaped perineum into two triangles. The anal triangle is the posterior part of the perineum and it contains the anal canal and anus. The urogenital triangle is the anterior part of the perineum and contains the urethra and the external genitalia. At the outset of dissection, it is important to understand that the pelvic diaphragmseparates the pelvic cavity from the perineum
The order of dissection will be as follows: Dissection of the anal triangle will begin with removal of skin covering the ischioanal fossa. The nerves and vessels of the ischioanal fossa will be dissected. The fat will be removed from the ischioanal fossa to reveal the inferior surface of the pelvic diaphragm.
The ischioanal (ischiorectal) fossa is a wedge-shaped area on either side of the anus. The apex of the wedge is directed superiorly and the base is beneath the skin. The ischioanal fossa is filled with fat that helps accommodate the fetus during childbirth or the distended anal canal during the passage of feces. The ischioanal fat is part of the superficial fascia of this region. The goal of this dissection is to remove the fat and identify the nerves and vessels that pass through the ischioanal fossa.
SUPERFICIAL PERINEAL POUCH
Overview
The perineum is a diamond-shaped area between the upper thighs and between the lower parts of the buttocks. It consists of structures that constitute the region below the pelvic floor. The perineum is bounded by the pubic symphysis anteriorly, the coccyx posteriorly, and the two ischial tuberosities laterally. Between the pubic symphysis and ischial tuberosities are the rami of the pubic bone and the rami of the ischia (since it is difficult to demarcate where the inferior ramus of the pubic bone ends and the ischial ramus begins, it is common parlance to call these structures “ischiopubic rami”). Extending from the ischial tuberosities to the coccyx (and sacrum) are the sacrotuberous ligaments. For descriptive purposes, the diamond-shaped perineum is divided into two triangles. The anal triangle is the posterior part of the perineum and it contains the anal canal and anus and has already been dissected. The urogenital triangle is the anterior part of the perineum and contains the urethra and the external genitalia. At the outset of dissection, it is important to understand that the pelvic diaphragm separates the pelvic cavity from the perineum.
Surface Anatomy of the Male Genitalia
Because of the variability of the genitalia of the cadavers, and the fact that as the dissection of this area progresses the surface anatomy of the genitalia will be compromised, a male and female cadaver will be placed in the dissection laboratory specifically for surface anatomy study.
- Penis
- Prepuce (foreskin) - note: if your cadaver has a circumcised penis, it will not have a prepuce. Find a cadaver that does. We will write the numbers of the lucky tables on the blackboards.
- Frenulum of the prepuce
- Glans of the penis
- Corona of the penis
- External urethral meatus (orfice)
- Body of the penis
- Dorsum of the penis
By convention, in the anatomical position, the penis is erect. The surface of the penis that is closest to the anterior abdominal wall is the dorsal surface of the penis. The penis is composed of (a) the root(consisting of the bulb and two crura), which is attached to the urogenital diaphragm and pubic arch; (b) the body (or shaft) formed by three cylindrical masses of erectile tissue: two corpora cavernosa penis and the corpus spongiosum penis; and © a terminal enlargement, the glans penis, which is a conical expansion of the corpus spongiosum over the ends of the corpora cavernosa. From their attachments on the ischiopubic ramus the two crura converge anteriorly as the corpora cavernosa penis and are joined by the corpus spongiosum (the distal continuation of the bulb of the penis). Study a drawing of a transverse section of the penis (L. penis, tail). Thesuperficial fascia of the penis (dartos fascia) has no fat, and contains the superficial dorsal vein of the penis. The deep fascia of the penis (Buck’s fascia) is an investing fascia. Contained within the deep fascia of the penis are the corpus spongiosum, corpus cavernosum (paired), deep dorsal vein of the penis (which is unpaired), dorsal artery of the penis (paired), and dorsal nerve of the penis (paired).
The deep perineal pouch (space) will not be dissected. The deep perineal pouch / urogenital diaphragm is a fibromuscular structure sandwiched between two fascial layers: the inferior fascia of the UG diaphragm (perineal membrane) and a similar layer, the superior fascia of the UG diaphragm. These facial sheets stretch horizontally across the pubic arch and attach to the anterior portion of the ischiopubic rami. The contents of the deep perineal pouch in the male include the membranous urethra, external urethral sphincter muscle, bulbourethral glands, the dorsal artery of the penis (a branch of the internal pudendal artery), and the dorsal nerve of the penis (a branch of the pudendal nerve).
