62 



CONSCIOUS NERVOUS OPERATIONS 



Nerve Fiber 



lemma (the inclosing membrane of the nerve fiber) becomes 

 continuous with the sarcolemma, and the axis cylinder of 



the nerve fiber branches many 

 times, the ramifications end- 

 ing in a flat or branched layer 

 ^ protoplasm containing nu- 

 clei. These terminal nerve 

 organs are called end plates 

 (Figs. 43 and 44). 



73. How a Muscle Contracts. 



Fig. 43. -Nerve endings in stri- ~ A muscle contracts or be- 



ated muscle fibers. comes shorter in proportion to 



its length, without change in 



bulk, under the influence of stimulus. It is the office of 

 the nerves of the muscles to carry to them their natural 

 stimulus, but muscles also 

 contract under the action 

 of other stimuli; for in- 

 stance, in consequence of 

 a sudden blow or pinch, 

 when heat is applied sud- 

 denly, when certain chemi- 

 cal substances are dropped 

 upon them, or when an 

 electric shock is conducted 

 to them. 



In living animals the 

 muscles are always more 

 or less contracted. This 



Fig. 44. Motor nerve endings in 

 striated muscle fibers. 



is due to the nervous influence which they constantly 

 receive. If their nerves are cut or destroyed, the mus- 

 cles lengthen. This tension of the muscles keeps them 

 ready for immediate action. When a nervous impulse 



