Lab 10 Pre-Lab Exercise:

1. Surface anatomy of the pelvis and perineum
The ability to recognize the normal appearance of structures in the perineum is an essential part of a physical examination. Palpable boney features of the pelvis can: 1) locate soft tissue structures, 2) help visualize the orientation of the pelvic inlet, and 3) help define the margins of the perineum.

The female pelvic girdle is relatively wider and shorter than that of the male, related to its additional roles of bearing the weight of the gravid uterus in late pregnancy and allowing passage of the fetus through the pelvis outlet during childbirth (parturition). In general, the pelvis is thicker and heavier in males (more robust), while the pelvic inlet is more heart shaped and narrower in males and more oval/rounded and wider in females. In addition, the sacrum and coccyx is more curved in males, the pelvic outlet is comparatively small, the pubic and subpubic arch is narrower (wider in females), the obturator foramen is round (more oval in females), and the acetabulum is large (smaller in females).


2. Orientation of the pelvis and perineum
In anatomical position, the anterior superior iliac spines (ASIS) and the anterior superior edge of the pubic symphysis lie in the same vertical plane. The pelvic inlet faces antero-superiorly. The urogenital triangle of the perineum is oriented in an almost horizontal plane and faces inferiorly, whereas the anal triangle is more vertical and faces posteriorly.




3. Margins of the perineum
The pubic symphysis, ischial tuberosities, and tip of the sacrum are palpable on patients and can be used to define the boundaries of the perineum. This is best done when the patient is lying on their backs with their thighs flexed and abducted in the lithotomy position. The ischial tuberosities are palpable on each side (large bony masses) near the gluteal fold (between thigh and gluteal region). They mark the lateral corners of the diamond shaped perineum. The tip of the coccyx is palpable at the midline posterior to the anal aperture and marks the posterior limit of the perineum. The anterior limit is the pubic symphysis.

The anal triangle is the posterior half of the perineum. The base of the triangle faces anterior and is an imaginary line joining the two ischial tuberosities. The apex of the triangle is the tip of the coccyx, and the lateral margins are the lines joining the coccyx to the ischial tuberosities. Major feature of the anal triangle is the anal aperture in the center of the triangle.

The urogenital triangle is the anterior half of the perineum. The base of the triangle faces posteriorly by an imaginary line joining the two ischial tuberosities. The apex of the triangle is the pubic symphysis. Lateral margins are line joining the pubic symphysis to the ischial tuberosities (overlying the pubic rami, which can be palpated). In women, the content of the triangle are the clitoris, the vestibule, and skin folds that together form the vulva. In men, the urogenital triangle contains the root of the penis.


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