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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The physiological complexity of the elements in any part of the central 

 system, either when different portions of the system from the same animal or 

 when the corresponding portions of different animals are compared, depends 

 on the number of paths by which the impulses are brought to the discharging 

 cells. 



Composite Character of Incoming - Impulses. — To these conclusions 

 based on the anatomy are to be added others suggested by clinical observa- 

 tions. In order that a patient suffering from a lesion between the visual and 

 motor ureas may be able to recognize an object and to indicate its use, it is 

 sometimes necessary that the object shall appeal to several senses. For 

 example, the name and use of a knife, when seen alone, may not be recalled, 



Fig. 118. — Schema showing in a purely formal manner the different sort of afferent impulses which may 

 influence the discharge of a cortical cell. 



but when it is taken into the hand — that is, when the dermal and muscular 

 sensations are added to the visual on< — the response is made, though, acting 

 alone, any one -et of sensations is inadequate to produce this result. 



Just where the block occurs in such a case it is not possible to say with 

 exactness, hut the lesion lies, as a rule, between the sensory and motor areas 

 concerned, and by the damage to the pathway, it is assumed that one or more 

 groups of impulses are so reduced in intensity that they are alone insufficient 

 to produce a reaction ; and therefore it is only when the impulses from several 

 sources are combined that a response can he obtained. 



Variations in Association. — It is a familiar fact that individuals differ 



