CHAPTER VII 



THE NEUROMUSCULAR MECHANISM 



THE most conspicuous manifestations of the activity 

 of the nervous system are the contractions of the skeletal 

 muscles. Locomotion, balancing, movements of the arms 

 and hands, changes of facial expression, speech all these 

 are examples. So also is breathing, a rhythmic process 

 which we are apt to regard as similar in its nature to the 

 beating of the heart, but which is fundamentally differ- 

 ent, inasmuch as every breath is the sign of an outflow of 

 nerve-impulses from the central gray matter, while the 

 heart beats as a result of its own intimate properties. 



Each skeletal muscle has a supply of efferent fibers. 

 It has also afferent connections which are of much im- 

 portance, though usually overlooked. A large muscle, 

 like the biceps in the upper arm, can be recognized to 

 consist of numerous parallel bundles set apart by connect- 

 ive tissue. These bundles, in their turn, can be resolved 

 into more slender prismatic subdivisions, and the finest 

 visible "grain" is perhaps the indication of the fibers which 

 are the working units of the mechanism. A typical 

 muscle-fiber is of hair-like proportions, being an inch or 

 two in length and, on an average, about 5^ inch in 

 diameter. Each of these fibers may be regarded as a 

 muscle in miniature and has its own nervous connection. 



It must not be inferred that the number of motor fibers 

 in the nerves is equal to the number of muscle-fibers sup- 

 plied. A single nerve-fiber, as it approaches its termina- 

 tion within the muscle, branches rather freely and is thus 

 linked to a considerable number of muscle-fibers. It 

 may govern as many as one hundred or the number may 

 be much smaller. Muscles which are under the most 



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