516 MANUAL OF PHYSIOLOGY. 



cells withhold the transmission of a stimulus, or lessen the im- 

 pulse reflected so as to produce little or no effect ; this is called 

 inhibition. 



5. Automatism. Nerve cells are supposed to have the power 

 of originating impulses ; e.g., those carrying on operations which 

 require to be of a more or less permanent kind, such as the closure 

 of the sphincter muscles and the partial contraction of the muscle 

 cells of the arteries. Automatic actions are sometimes spoken of as 

 those that are continuous and those that undergo rhythmical 

 changes. If carefully examined, however, most of the so-called 

 constant automatic nervous actions will be found to show some 

 traces of rhythmic relaxation. Nerve cells, with automatic 

 properties, may be exercised in preventing reflex actions having 

 their full effect, and thus they act as aids to the controlling 

 part played by the reflex cells. And in the same way automatic 

 cells may be influenced and even regulated by impulses coming 

 from the periphery to reflex centres in the vicinity, which join 

 forces with the automatic centre. Thus the act of respiration 

 may be performed by pure automatism, and its centre supplies 

 a good example of an automatic group of cells. As a matter of 

 fact, however, the respirations are regulated by a reflex mechanism, 

 the channels of which reside in the vagus nerve. 



Moreover it is in the nerve cells that we must seek mental 

 activity, under which term may be considered perception, volition, 

 thought, and memory. It is very difficult to allocate the due pro- 

 portions of reflection, coordination, augmentation, inhibition, au- 

 tomatism, etc., requisite for the development of what we must call 

 mental faculties, but there can be no doubt that the mind must 

 be the resultant of a long series of external and internal excita- 

 tions, modified by intrinsic influence, and acting upon innumerable 

 groups and masses of nerve cells, the general outline of which has 

 been rough-hewn by hereditary tendency. 



