694 NERVOUS SYSTEM OF THE HORSE 



such rami. A nerve may be connected with two ganglia, and a ganghon may be 

 connected with two nerves. 



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

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

 conveys impulses toward the periphery. Its fibers are axones of the large cells 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. 431, 433, 518, 523) number eight 

 pairs. Of these the first 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 vertebrae. 



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

 the neck. They divide usually into inner and outer branches. The internal 

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

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

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

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

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

 plexus. 



Tlie 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 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 (antero-internal) 

 foramen of the atlas. Its dorsal branch (N. occipitalis) passes upward and outward 

 between the obliqui capitis and the recti capitis dorsales and supplies branches to 

 these muscles, the seutularis and posterior auricular muscles, and the skin of the 

 poll. The ventral branch descends through the alar (antero-external) foramen of 

 the atlas, crosses over the vcnitral 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 l)ehind the intermediate tendon. 

 In the recessus atlantis the ventral ])ranch is connected by one or more twigs with 

 the superior cervical ganglion of the sympathetic, and a little lower with the hypo- 

 glossal nerve. It also sends branch(»s to the ventral straight muscles of the head 

 and the thyro-hyoid(ms. Below th(> atlas the ventral branch is crossed super- 

 ficially by the spinal accessory nerve, the occipital artery, and the inferior 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 as('(>nds Ix^tween the complexus and the ligamentum 

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

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

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



