812 NERVOUS SYSTEM OF THE HOKSE 



The dorsal root is sensory or afferent, i. e., it conveys impulses to the central system. Its 

 fibers are axones of the ceUs of the spinal ganglion. The ventral root is motor or efferent, and 

 conveys impulses toward the periphery. Its fibers are axones of the large ceUs in the ventral 

 gray columns of the spinal cord. The common trunk or nerve formed by the union of the two 

 roots contains both kinds of fibers, as do also their primary divisions. In addition to these fibers, 

 which are distributed to the skeletal muscles and the skin, the spinal nerves contain fibers derived 

 from the sympathetic system through the rami communicantes; these go to the glands and 

 unstriped muscle and are designated secretory and vasomotor fibers. 



THE CERVICAL NERVES 



The cervical nerves (Nervi cervicales) (Figs. 556, 558, 650, 655) number eight 

 pairs. The first of these emerges through the intervertebral foramen of the atlas, 

 the second through that of the axis, and the eighth between the last cervical and 

 the first thoracic vertebra. 



The dorsal branches are distributed to the dorso-lateral muscles and skin of 

 the neck. They divide usually into lateral and medial branches. The medial 

 branches (Rami mediales) run in general across the multifidus and the lamellar part 

 of the ligamentum nuchse to the skin of the dorsal border of the neck; they supply 

 the deep lateral muscles and the skin. The lateral branches (Rami laterales) are 

 chiefly muscular in their distribution. The dorsal branches of the third to the 

 sixth nerves are connected by anastomotic branches to form the dorsal cervical 

 plexus. 



The ventral branches are smaller than the dorsal ones — an exception to the 

 general rule. They increase in size from first to last. They supply in general the 

 muscles and skin over the lateral and ventral aspect of the vertebrae, but the last 

 three enter into the formation of the brachial plexus, and the two or three preceding 

 the last give off the roots of the phrenic nerve. An irregular ventral cervical plexus 

 is formed by anastomoses established between the ventral branches. The following 

 special features may be noted: 



The first cervical nerve emerges through the intervertebral foramen of the 

 atlas. Its dorsal branch (N. occipitalis) passes dorso-laterally between the obliqui 

 capitis and the recti capitis dorsales and supplies branches to these muscles, the 

 scutularis and posterior auricular muscles, and the skin of the poll. The ventral 

 branch descends through the alar foramen of the atlas, crosses over the ventral 

 straight muscles and the carotid artery under cover of the parotid gland, and divides 

 into two branches. The anterior branch enters the omo-hyoideus muscle. The 

 posterior branch passes downward and backward under cover of that muscle, unites 

 with a branch of the ventral division of the second cervical nerve, and continues its 

 course on the ventro-lateral surface of the trachea to enter the sterno-thyro-hyoideus 

 behind the intermediate tendon. In the recessus atlantis the ventral branch is 

 connected by one or more twigs with the anterior cervical ganglion of the sym- 

 pathetic, and a little lower with the hypoglossal nerve. It also sends branches to 

 the ventral straight muscles of the head and the thyro-hyoideus. Below the atlas 

 the ventral branch is crossed superficially by the spinal accessory nerve, the occi- 

 pital artery, and the ventral cerebral vein. 



The second cervical nerve is larger than the first. It emerges from the verte- 

 bral canal through the intervertebral foramen of the anterior part of the arch of the 

 axis. Its dorsal branch ascends between the complexus and the ligamentum 

 nuchse and ramifies in the skin of the poll. The ventral branch gives off muscular 

 branches to the rectus capitis ventralis major, and anastomotic branches to the 

 spinal accessory and the ventral divisions of the first and third cervical nerves; 

 one of these crosses over the carotid artery and concurs in the formation of the nerve 

 to the sterno-thyro-hyoideus mentioned above. The ventral branch then becomes 

 superficial by passing between the two parts of the brachiocephalicus, and divides 

 into posterior auricular and cutaneous branches. The posterior auricular nerve 



