CENTRAL NERVOUS SYSTEM. 659 



place is not the shortest one between the two groups of cells, but a path in 

 which the actual crossing of the impulses occurs toward the cephalic end of the 

 cord, so that they must pass up the cord on one side and down on the other. 



Strength of Stimulus. In a reflex response the strength of the stimulus 

 influences the extent to which the muscles are contracted ; the number of 

 muscles taking part in the contraction, and the length of time during which 

 the contraction continues. That the strength of the stimulus influences the 

 extent to which the contraction of a given group of muscles takes place is 

 easily shown when, for example, the toe of a reflex frog which has been sus- 

 pended is stimulated by pinching it or dipping it in dilute acid. In this case, 

 if the stimulus be slight, the leg is but slightly raised, whereas if the stimulus 

 be strong it is drawn up high. In the same way by altering the stimulus the 

 muscles which enter into the contraction may be only those controlling the 

 joints of the foot, whereas, with stronger stimuli, those for the knee and hip 

 are successively affected, thereby involving a much larger number of muscles. 

 Here, too, we infer a spread of the incoming impulses which is orderly, since 

 the several joints of the limb are moved in regular sequence. 



The responses which are thus obtained are not spasmodic, but are contrac- 

 tions of muscles in regular series, giving the appearance of a carefully co- 

 ordinated movement a movement that is modified in accordance both with 

 the strength of the stimulus and its point of application. Moreover, such a 

 movement may occur not only once but a number of times, the leg being 

 alternately flexed and extended during an interval of several seconds, although 

 the stimulus is simple and of much shorter duration. 



Continuance of Response. The continuance of the response after the 

 stimulus has been withdrawn must be of course the result of a long-continued 

 chemical change at some point in the pathway of the impulse, and it appears 

 probable by analogy with the results obtained from the direct stimulation of 

 the central cortex, that in these cases the stimulating changes are taking place 

 in the central cells. 



Latent Period. It has been observed that in the case of a reflex frog an 

 interval of varying length elapses between the application of a stimulus and 

 the appearance of a reaction. The modifications of the interval according to 

 variations in the stimulus have been carefully studied. When dilute acid is 

 used as a stimulus, this latent interval decreases as the strength of the acid is 

 increased. When separate electrical or mechanical stimuli are employed, the 

 reaction tends to occur after a given number of stimuli have been applied, 

 although the time intervals between the individual stimuli may be varied 

 within wide limits. The experimental evidence for electrical stimuli shows 

 that the time intervals may range between 0.05 second and 0.4 second, 1 while 

 the number of stimuli required to produce a response remains practically con- 

 stant. 



Summation of Stimuli. A single stimulus very rarely if ever calls 

 forth a reaction if the time during which it acts is very short, and hence there 

 1 Ward : Archiv fiir Anatomic und Physiologic (Physiol. Abthl.), 1880. 



