THE PERIPHERAL NERVOUS SYSTEM. 383 
the foramen magnum. After receiving the rootlets from the 
medulia, 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 interconnect it with the vagus, sympathetic, and 
hypoglossal. It then turns laterad and caudad (Fig. 158, 1) 
and pierces the cleidomastoid muscle (@), 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 scapulz ventralis, sends branches 
to the clavotrapezius, and may be traced to the acromiotrape- 
zius and spinotrapezius, which it supplies. 
XII. N. HypoGLossus (Fig. 156, 4).—The twelfth or 
hypoglossal nerve arises from the ventral side of the medulla 
(Fig. 138, X/Z/), 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 ¢), 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 (7), and divides into two branches, one (g) to 
the thyrohyoid muscle (9), the other to the sternohyoid (8) 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 
