206 ELEMENTS OF MAMMALIAN ANATOMY. 



nerve bundles, forming what are known in the brain region 

 as the cranial nerves and in the region of the cord as the 

 spinal nerves. 



The Cranial Nerves. There are twelve pairs of cranial 

 nerves, all of which pass through foramina in the base of 

 the skull, and all except one, the tenth or vagus, are dis- 

 tributed 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. 93). 



The olfactory, optic and auditory are the only cranial 

 nerves wholly sensory. The oculomotor, patheticus or 

 trochlearis, abducens, spinal accessory, and hypoglossal are 

 wholly motor. The trigeminal, facial, glossopharyngeal 

 and vagus contain both motor and sensory fibers. 



The dissection of the cranial nerves is very difficult. A 

 head, containing a brain hardened by a formalin injection, 

 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 twenty-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. 66). 



Some of the sensory nerve roots bear ganglia, the largest 

 of which is the Gasserian ganglion, more than a half centi- 

 meter in diameter, forming a knot on the sensory root of 

 the trigeminal, within the cranial cavity (Fig. 93). 



