There are five sacral nerves. The first four pass out of the spinal canal by the sacral foramina, and the fifth by the opening between the last sacral foramen and the first coccygeal bone. After emerging from the spine, they each divide into a superior and an inferior branch. The former of these pass upwards through the supra-.sacral openings, and are distributed to the muscles on the side of the sacral spine and to the skin of the croup. The lower branches are much the larger of the two, and pass in a direction downwards and backwards, to be distributed as follows: - The first and second, by joining with the third, fourth and fifth lumbar, form the lumbo-sacral plexus.
The third forms the internal pudic nerve, which winds round the ischial arch, and with its fellow on the opposite side gains the dorsal border of the penis, along which it runs, and ends in the mucous membrane covering the glans penis. In its course from behind forward it gives numerous branches to the corpora cavernosa and the urethral canal, and while in the pelvis, two small ramuscules go to the perineal region, and hsemorrhoidal branches to the anus.
The fourth, together with a branch of the third, innervates the sphincter muscle of the anus and surrounding skin, and the fifth, after giving a branch to the first coccygeal nerve, is expended in the muscles and integument about the base of the tail and the root of the penis.