THE NERVOUS SYSTEM 23 1 



known as reflex action. If, however, the impulse travels along 

 the fiber ir, and thence through the fiber s to the brain, whence 

 an impulse descends through the fiber t' , ts, the process is known 

 as voluntary reaction. 



LABORATORY STUDIES AND SUGGESTIONS 



1. Describe the membranes of the cord. 



2. What diflference is there in the size of the nerve roots in the various regions 

 of the cord? 



3. Draw a cross-section of the cord showing all features visible to the naked 

 eye. 



4. How does the arrangement of the gray matter of the cord and brain dififer? 



5. Describe the processes of nerve cells. 



6. In what portions of the body are nerve cells found? 



7. Procure a piece of spinal cord from the butcher-shop. Smear a bit of the 

 gray matter on a glass slip, dry, then stain in hematoxylin, wash, and after 

 drying mount in balsam. Draw and describe nerve cells thus found. 



THE PERIPHERAL NERVES 



All portions of the head, trunk, and limbs of the cat are 

 supplied with nerve fibers which are in communication with 

 the central nervous system by means of fifty-two pairs of 

 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 distributed 

 to structures of the head and neck.. They are divided accord- 

 ing 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 and other portions of 

 the fore-parts of the head with sensory fibers (Fig. 104). 



The olfactory (I), optic (II), and acoustic (VIII) are the only 

 cranial nerves wholly sensory. The oculomotor (III), troch- 



