516 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



EEGUl-ATION OF MOVEMENTS BY NERVOUS SYSTEM — VOLUNTARY 



MOVEMENTS. 



The most conspicuous part played by the nervous system in the 

 phenomena of life is that which produces and regulates the general 

 movements of the body — movements brought about by the so-called 

 voluntary muscles. These movements are actually the result of 

 impressions imparted to sensory or afTerent nerves at the periphery, 

 e. g., in the skin or in the several organs of special sense; the effect 

 of these impressions may not be immediate, but can be stored for an 

 indefinite time in certain cells of the nervous system. The regu- 

 lation of movements — whether they occur instantly after reception 

 of the peripheral impression or result after a certain lapse of time; 

 whether they are accompanied by conscious sensation or are of a 

 purely reflex and unconscious character — is an intricate process, and 

 the conditions of their coordination are of a complex nature, involving 

 not merely the causation and contraction of certain muscles, but also 

 the prevention of the contraction of others. For our present knowl- 

 edge of these conditions we are largely indebted to the researches of 

 ^Prof. Sherrington, 



INVOLUNTARY MOVEMENTS EFFECTS OF EMOTIONS, 



A less conspicuous but no less important part played by the nervous 

 system is that by which the contractions of involuntary muscles are 

 regulated. Under normal circumstances these are always inde- 

 pendent of consciousness, but their regulation is brought about in 

 much the same way as is that of the contractions of voluntary 

 muscles, viz, as the result of impressions received at the periphery. 

 These are transmitted by afferent fibers to the central nervous 

 system, and from the latter other impulses are sent down, mostly 

 along the nerves of the sympathetic or autonomic system of nerves, 

 which either stimulate or prevent contraction of the involuntary 

 muscles. Many involuntary muscles have a natural tendency to 

 continuous or rhythmic contraction, which is quite independent of the 

 central nervous system. In this case the effect of impulses received 

 from the latter is merely to increase or diminish the amount of such 

 contraction. An example of this double effect is observed in con- 

 nection with the heart, which, although it can contract regularly and 

 rhythmically when cut off from the nervous system and even if 

 removed from the body, is normally stimulated to increased activity 

 by impulses coming from the central nervous system through the 

 sympathetic, or to diminished activity by others coming through the 

 vagus. It is due to the readiness by which the action of the heart is 

 influenced in these opposite ways by the spread of impulses generated 

 during the nerve storms which we term "emotions" that in the 



