COORDINATION 



71 



of contraction is determined by the strength of the nervous 

 impulse, a strong impulse producing greater contraction 

 than a weak impulse. In health the muscle fibers are the 

 obedient servants of the nerves, and if they act in a pur- 

 poseful and coordinated manner, it is because the nerves 

 stimulate them to act in this way. The explanation of 

 purposeful and coordinated action must therefore be sought 



FIG. 



Cross section of a nerve 



Showing five bundles of nerve fibers bound together by connective tissue con- 

 taining a few blood vessels. On the right are shown four fibers more highly 

 magnified, the dark center being the ax on, around which is the white or fatty 

 sheath, both axon and fatty sheath being inclosed within the fine membrane, 

 the neurilemma. Cf . Fig. 40 



not in the muscles but, behind these, in the nervous system, 

 to the study of which we now turn. 



2. Structure of a nerve. A nerve, like a muscle, may be 

 separated into long fibers (Fig. 40) which are bound together 

 by connective tissue containing blood vessels, lymph spaces, 

 and lymphatics. The nerve fiber, which is the essential 

 part of the nerve, just as the muscle fiber is of the muscle, 

 differs somewhat in structure in different nerves; it generally 



