166 PRINCIPLES OF VETERINARY SCIENCE 



The eighth or auditory nerve goes to the ear, and has to do with 

 the special sense of hearing. 



The ninth or glossopharyngeal nerve is mixed in character, and 

 conveys both motor and sensory impulses to the tongue and 

 pharynx. 



The tenth, vagus or pneumogastric nerve, is a very large nerve- 

 trunk with both motor and sensory fibers that supply the stom- 

 ach, heart, lungs, trachea, pharynx, and related organs. 



The eleventh or spinal accessory nerve has motor fibers only. 



The twelfth or hypoglossal nerve is purely motor and innervates 

 the muscles of the tongue. 



The spinal cord is situated in the vertebral canal, and extends 

 from the medulla, which it continues, to about the middle of the 

 sacrum. The cord is enlarged where the nerves to the limbs are 

 connected. On cross-section a dorsal median septum and a 

 ventral median fissure are seen, also the central canal of the cord 

 (Fig. 54) . White matter surrounds a core of gray matter. The 

 latter is arranged in the form of a capital H. Dorsal and ventral 

 gray horns, which give rise to nerve-roots with the same names, 

 are plainly visible. 



The spinal nerves number forty-two pairs. They take origin 

 from the spinal cord and leave the vertebral canal through the 

 intervertebral foramina. Each is connected to the cord by a 

 dorsal and a ventral root. On the dorsal root is found the spinal 

 nerve ganglion composed of nerve cells. These cells are sensory 

 in character and bipolar, that is, have two branches, one to the 

 periphery and one to the spinal cord. The nerves of the ventral 

 root are motor in character, having no ganglia, their cells being 

 located in the spinal cord. It is evident that a spinal nerve, 

 beyond the point of union of its two roots, is mixed in character, 

 containing both sensory and motor fibers. 



The sympathetic nervous system is composed of two chains 

 of ganglia which are located one on each side of the vertebral 

 column, external to the spinal canal. The nerve-fibers of this 

 system come from the sensory nerve-roots of the spinal nerves. 

 At intervals communicating branches join it with the central 

 nervous system. There are special ganglia and plexuses for the 

 main groups of visceral organs. The most important is the 

 solar plexus, which is situated on the dorsal wall of the abdominal 

 cavity in contact with the aorta. 



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