THE PERIPHERAL NERVOUS SYSTEM. 383 



the foramen magnum. After receiving the rootlets from the 

 medulla, it leaves the cranial cavity along with the vagus and 

 glossopharyngeal by the jugular foramen. Just outside the 

 foramen (Fig. 156, c) it becomes involved in a plexus of fine 

 branches which intercotinect it with the vagus, sympathetic, and 

 hypoglossal. It then turns laterad and caudad (Fig. 158, 1) 

 and pierces the cleidomastoid muscle (d), to which it gives 

 small branches. It then divides: one branch enters the 

 sternomastoid muscle ; the other passes caudad along the 

 dorsal border of the levator scapulae ventralis, sends branches 

 to the clavotrapezius, and may be traced to the acromiotrape- 

 zius and spinotrapezius, which it supplies. 



XII. N. IIYPOGLOSSUS (Fig. 156, /;). — The twelfth or 

 hypoglossal nerve arises from the ventral side of the medulla 

 (Fig. 138, XII), as previously described. It emerges from 

 the skull through the hypoglossal canal. At first it passes 

 ventrad, then gradually turns craniad, following thus a curved 

 course and passing successively laterad of the vagus and sym- 

 pathetic nerves (Fig. 156, d and e), the common carotid artery, 

 and the cranial cornu of the hyoid bone. A short distance 

 from its origin the nerve gives off a ramus descendens (Fig. 

 156,/), which receives a communicating branch from the first 

 cervical nerve (J), and divides into two branches, one {g) to 

 the thyrohyoid muscle (9), the other to the sternohyoid (s) and 

 sternothyroid muscles (7 ). The main nerve passes beneath the 

 mylohyoid muscle into the tongue, at first following the lingual 

 artery, then laterad of it, then crossing it again. It sends 

 branches to all the muscles of the tongue, and may be traced 

 to its tip. 



The hypoglossal nerve is involved in the plexus just outside 

 of the jugular foramen, with which the vagus, sympathetic, and 

 accessory nerves are also connected. 



2. Spinal Nerves. — The origin and general features of the 

 spinal nerves have been described in connection with the 

 account of the spinal cord (page 337). A description of their 

 peripheral distribution will now be given. 



A. Cervical Nerves. — There are eight pairs of cervical 

 nerves. The first leaves the vertebral canal through the 



