4. N. pudendus ([Fig. 162], l; [Fig. 112], 8, [page 265]).—This arises ([Fig. 162], l) from the sacral plexus caudad of the great sciatic nerve. It has two roots, one from the second sacral, one from the third, and it may also receive a small accession from the great sciatic nerve (h) three or four centimeters caudad of the origin of the latter. It passes at first dorsolaterad, laterad of the coccygeus muscle, then curves mediad toward the anus. It divides into two branches: one passes to the base of the penis, sends twigs into the compressor urethræ muscle, and passes onto the dorsal surface of the penis. This branch is N. dorsalis penis; it extends with the artery of the same name to the glans. The second branch of N. pudendus (N. hemorrhoidalis medius) passes to the anus and innervates the muscles and other structures about the caudal end of the rectum. In the female, branches of N. pudendus supply the urogenital sinus and adjacent structures ([Fig. 112], 8).
5. N. cutaneus femoris posterior ([Fig. 162], k; [Fig. 163], h).—This arises from the sacral plexus, its roots coming chiefly from the second and third sacral nerves, and passes caudodorsad, at first in close connection with N. pudendus. It then accompanies the posterior gluteal artery and vein, sends branches (perineal nerves) into the fat at the sides of the anus, and branches onto the lateral surface of the biceps muscle. One of the latter follows the communicating vein ([Fig. 163], k′) from V. saphena parva, and may be traced as far distad as the popliteal space.
6. N. hemorrhoidalis inferior (or posterior) ([Fig. 162], m).—This arises by two roots, from the second and third sacral nerves, and passes with the inferior hemorrhoidal artery ventrad across the lateral surface of the rectum to the urethra. Here it divides into two parts: one passes craniad to the bladder, the other caudad onto the ventrolateral surface of the rectum.
A small nerve passes from the sacral plexus, especially from the third sacral nerve, into M. levator ani ([Fig. 162], 11). Another small nerve from the same region passes to M. coccygeus, and a third to the proximal end of M. tenuissimus.
E. Nerves of the Tail. Coccygeal Nerves.
—From the intervertebral foramina of the first seven or eight caudal vertebræ spinal nerves of the usual type are given off. The dorsal rami innervate the dorsal muscles of the tail. The ventral rami are interconnected with each other and with the last sacral nerve by a longitudinal cord; they innervate the muscles and integument of the ventral side of the tail.
3. The Sympathetic Nervous System. Systema nervorum sympathicum.
The sympathetic system consists essentially of a chain of ganglia on each side of the ventral surface of the vertebral column, interconnected by longitudinal nerve-cords, and stretching from the base of the skull to the tail. The ganglia are connected to the spinal nerves by communicating branches, and numerous branches pass from them to the abdominal and thoracic viscera, and to the walls of the lymph- and blood-vessels, forming complicated plexuses.
Cervical portion ([Fig. 156], [page 379]).—The sympathetic system begins just caudad of the tympanic bulla as the superior cervical ganglion (G. cervicale superius) ([Fig. 156], e). This is a large ganglion closely applied to the ganglion nodosum (d) of the vagus, and lying on its ventrocranial side. From the superior cervical ganglion the sympathetic trunk passes caudad usually closely bound up with the vagus (d′), so that the two can scarcely be distinguished. They lie (i) at the side of the trachea (6), close against the lateral surface of the carotid artery. A few centimeters before reaching the first rib the two separate ([Fig. 157], [page 381]); the sympathetic (c) is here the smaller and more dorsal of the two. Just craniad of the first rib the sympathetic usually forms a small swelling, the middle cervical ganglion ([Fig. 157], d), beyond which it divides into two portions. The larger dorsal division passes dorsocaudad and joins a large ganglion, the inferior cervical (e), which lies at the head of the first rib, on the lateral surface of the longus colli muscle. The ventral branch (g′) passes caudad, turns laterad about the medial and caudal surface of the subclavian artery, and likewise reaches the inferior cervical ganglion.
In some cases the middle cervical ganglion (d) is lacking.