CENTRAL NERVOUS SYSTEM. 



649 



occur : in the first place, the impulse reaches a far greater number of cells 

 than evidently discharge, and in the second, the pathway followed by the im- 



FIG. 169. Schema to show how, by means of the collaterals and the central cells, several paths are 

 open to any impulse coming in over A,A, also showing how an impulse may arrive at a given part of an 

 efferent cell by more than one pathway among the central cells : C,C, C'C'. C"C", neurons of central 

 <;ells the bodies of which are located in other segments ; E, efferent cell. 



pulses which do produce the discharge is by no means the only pathway over 

 which the impulses can or do travel. 



The most convenient illustration of this process of diffusion can be obtained 

 by a study of the knee-kick or knee-jerk as it is more commonly called. 

 The reaction in question consists in a contraction of extensor muscles of the 

 knee in consequence of a blow on the tendon just below the knee-pan. As 

 a result of this contraction, the leg is extended, and a kick of greater or Jess 

 extent is made from the knee joint. Very careful studies of the conditions 

 controlling this response have been made by a number of investigators, 

 notably Westphal, 1 Lombard, 2 Bowditch and Warren, 3 Weir-Mitchell, 4 



1 Archiv filr Psychiatric, 1875. 

 a Journal of Physiology, 1890. 



2 American Journal of Psychology, 1887. 

 4 Philadelphia Medical News, Feb., 1886. 



