PART IX 



THE CENTRAL NERVOUS SYSTEM AND THE CONTROL 

 OF MUSCULAR ACTIVITY 



(Contributed by A. C. REDFLELD) 



CHAPTER LXXXIX 

 THE EVOLUTION OF THE NEUROMUSCULAR MECHANISM 



Disease of the nervous system confronts the physician with a complex 

 group of symptoms, a syndrome due to more or less sharply localized 

 disturbances in its function. The province of physiology is to analyze 

 the fundamental activities of the nervous system and assign to various 

 parts a functional significance, so that the complicated picture presented 

 by disease may be recognized as the logical result of a lesion of definite 

 nature, location, and extent. Thus we find the symptoms or nervous dis- 

 ease grouping themselves as disturbances (1) of sensation (anesthesia, 

 etc.) (2) of movement, both volitional and reflex (paralysis, etc.) (3) 

 of postural coordination (muscular spasms and flaccidity, ataxia, etc.) 

 (4) of the mechanisms of integration in the nervous system including the 

 higher mental functions of association, memory, and attention, etc. 



Corresponding to each of these groups we can recognize a definite as- 

 pect of nervous activity, the successful performance of which depends 

 on the continuity of more or less precisely known groups of cells within 

 the nervous system, and disturbances of any of these aspects can be as- 

 signed to lesions affecting some part of the group of nerve cells on which 

 they depend. 



The fundamental function of the nervous system is to correlate the 

 activities of the body so that its many parts may act harmoniously and 

 as a unit in preserving the welfare of the individual. What the basis 

 of this integration of activity which manifests itself so remarkably in 

 the behavior of the higher animals, is, can best be illustrated by a consid- 

 eration of its evolutionary development. 



Primitive Neuromuscular Mechanisms 



In the unicellular organisms three processes occur when a response is 

 brought about by a stimulus. These are (1) excitation, or the. setting up 

 of a physiological disturbance; (2) conduction, or the spread of the dis- 



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