28 



ZOOLOGY 



just defined, but it is fortunate for us that they require 

 no effort of the mind to call them to activity. It must 

 also be said that even the typical voluntary muscles 

 carry on most of their work with, as it were, only 

 general instructions from the nervous centers. In walk- 

 ing or writing, for instance, we are wholly unaware of 

 the details of muscular movements, though we will the 

 operations in a large and general sense. Reflex centers, 

 uncontrollable by the will, often dominate the move- 

 ments of so-called voluntary muscles. 



6. Nerve tissue has to do with the conveyance of 

 stimuli along definite paths. The old primitive gen- 

 eralized response is modified in such a manner that 

 messages are flashed from the surface to the brain or 

 spinal cord, and thence back to the muscles of the part 

 affected by the stimulus, in much less time than it takes 

 to tell about it. Psychologists have determined by 

 actual experiment that the transmission is not in- 



Drawing by R. Weber 



FIG. 7. Diagram of a nerve cell. M = muscle-fiber. N = nucleus. The arrow 

 indicates the direction of the external stimulus. The disturbance set up is com- 

 municated along the nerve-fiber, as along a telegraph wire, to the muscle-fiber, 

 which thereupon contracts. The muscle is in the ulterior of the body, but is able 

 to react to events going on outside. The diagram illustrates a very simple type. 

 In the higher animals the usual course of events is different. The impulse is com- 

 municated to the brain or spinal cord, and the central nervous system sends out a 

 call through an efferent (out-carrying) nerve for action. Volition comes into play, 

 and reactions may be controlled by the will. Thus it is possible by an effort to 

 avoid sneezing. 



