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). 
