Part 2: Perineal Region

In this section, we will look at structures associated with the perineal region. Identify the terminal part of the large intestine, the rectum, as it passes through the pelvic region. The rectum will pass feces on through the anal canal and anus to be expelled from the body.

• Identify the external opening of the anal canal, which is the anus.

• Identify the anal sac (paranal sinus)- opening(s) in the cutaneous zone of the anal canal.

The anal canal is surrounded on either side by an internal anal sphincter m. (smooth m.; not easily seen) and an external anal sphincter m. (striated m.). Identify the external anal sphincter m. on your specimen. (See DG Figs. 4-45A, 4-47A, and 4-54)

• Dissection Note: If necessary, remove skin surrounding the anus/anal canal to observe the external anal sphincter m.

• Comment: Note that the external anal sphincter is innervated by the caudal rectal n., which is a branch from the pudendal n. These nerves will be discussed during the Application portion of the course. You do not need to dissect these nerves, but should view them on the demonstration specimens if visible.

Note that there is a pudendal n. that emerges caudolaterally from the pelvic cavity, near the pudendal vessels, medial to the superficial gluteal m. (See DG Figs. 4-65, 4-66, 4-68, and 4-69) While this nerve, and its branches, are particularly difficult to dissect, they are clinically relevant and should be mentioned. If possible, observe the pudendal n. and it’s branches that have been dissected out on one of the demonstration specimens.

• From the pudendal n., a branch called the caudal rectal n. courses to the external anal sphincter m. (which it supplies). (See DG Fig. 4-69)

• Note that there are also perineal nerves arising from the pudendal n. supplying the skin of the anus and perineum, and continuing to the scrotum/labium. (See DG Figs. 4-65, 4-66, 4-68, and 4-69)

• As you trace the pudendal n. distally, note the dorsal nerve of the penis (male)/dorsal nerve of the clitoris (female) following alongside/near an artery of the same name. Note that the dorsal nerve of the penis will be dissected in the Male Genitalia section of this chapter.

  • • Dissection Note: In the male, the dorsal nerve of the penis will be sizable and easy to trace, while the dorsal nerve of the clitoris in the female may be small and difficult to locate. (In the tomcat, the dorsal nerve of the penis will be located on the ventral surface of the penis (due to the caudally directed penis); you need not trace the dorsal nerve of the clitoris in the queen.)

Identify the rectococcygeus m. extending from the dorsal surface of the rectum to the ventral surface of the tail (ventral midline). (This muscle is smooth muscle and will appear lighter in color than striated muscles.) (See DG Figs. 4-45A, 4-47A, and 4-54)

Identify the levator ani m. and the coccygeus m. that make up the pelvic diaphragm. You should attempt to view these muscles from the lateral side (outside of the pelvis) as well as from the view within the pelvis you have opened up. (See DG Figs. 4-37, 4-45, 4-47, and 4-54)