Now we'll look at the rectum, where fecal material is stored before being eliminated. Looking down into the pelvis from above, we can only see the most proximal part of the rectum. Here it is. To see all of the rectum we'll look at two different specimens, one divided along this line, one along this line, going all the way down through the pelvis and perineum.
Before we look at the rectum, we need to get our bearings. On each side the cut went through the iliac crest, the blade of the ilium, the acetabulum, and the ischio-pubic ramus.
Here's the same cut made on a dry bone specimen. Here's the pelvic brim, and here's the pelvic brim in the dissection. Here's the cut edge of the peritoneum that lines the small upper part of the pelvic cavity. Here's the lower end of the sigmoid colon, here's the rectum.
From here, the peritoneum that covers the upper part of the rectum sweeps forwards, leaving most of the rectum with no peritoneal covering. We'll come back to this view in a minute.
To see what's behind the rectum and in front of it, we'll look at a different specimen that's been divided just to the left of the midline, leaving the rectum intact. Here's the promontory of the sacrum, here's the divided body of the pubis. Here's the rectum. Here's the lower limit of its peritoneal covering. The rectum lies in front of the lower half of the sacrum, and the coccyx. It conforms to their overall curve.
Here in its lower part the rectum runs downward and forward on this sling of muscle, the levator ani. The levator ani is the principal structure of an important partition, the pelvic diaphragm. The rectum turns sharply backwards as it passes through the pelvic diaphragm, becoming continuous with the anal canal.
In front of the rectum are the bladder, the seminal vesicles, and the prostate in the male, and the uterus and vagina in the female.
To see some more details, we'll go to the front view again. As the sigmoid colon merges into the rectum, the teniae broaden out, so that the rectum has a coat of longitudinal muscle that’s almost continuous.
The lower part of the rectum, known as the ampulla, is quite distensible. This space around the rectum isn't empty. In life it's occupied by fatty connective tissue that accommodates to changes in the size and shape of the rectum.
Here's the pelvic diaphragm, sloping down on each side. It's formed mainly by the levator ani muscle. This is the puborectalis part of the levator.