2l8 ELEMENTS OF MAMMALIAN ANATOMY. 



distributed to structures of the head and neck. They 

 are divided according to function into motor and sensory. 

 Some of the nerves communicate with the brain by more 

 than one root, and in such cases the same nerve may have 

 sensory fibers in one root and motor fibers in another. 

 For example, the trigeminal nerve transmits a stimulus 

 causing the muscles of mastication to contract and also 

 supplies the teeth with sensory fibers (Fig. 90). 



The olfactory, optic, auditory and glossopharyngeal 

 are the only cranial nerves wholly sensory. The oculo- 

 motor, patheticus or trochlearis, abducens, facial, spinal 

 accessory, and hypoglossal are wholly motor. The 

 trigeminal and pneumogastric contain both motor and 

 sensory fibers. 



The dissection of the cranial nerves is very difficult. 

 A head, containing a brain hardened by a formalin in- 

 jection, should be placed in 500 c.c. of 5% nitric acid, 

 which will decalcify the bone in about a week. After 

 washing out the acid by soaking the specimen in running 

 water tw^enty-four hours, the dissector may with much 

 care follow the course of the nerves peripherad from their 

 origin at the base of the brain. The vagus nerve must 

 of course be traced in an entire specimen, where it may 

 be easily followed in the neck region along with the 

 carotid artery, whence it passes to the lungs and stomach 

 (Fig. 64). 



Some of the sensory nerve roots bear ganglia, the 

 largest of which is the Gasserian ganglion, more than 

 a half centimeter in diameter, forming a knot on the 

 sensory root of the trigeminal, within the cranial cavity 

 (Fig. 90). 



The Spinal Nerves.- -There are forty pairs of nerves 

 connected with the spinal cord. They issue from the 

 vertebral canal through the intervertebral foramina. 



