CENTRAL NERVOUS SYSTEM. 211 



The responses which are thus obtained arc not spasmodic, but are con- 

 tractions 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 move- 

 ment may occur not only once, but a number of times, the leg being alter- 

 nately 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 stimulus has been withdrawn must be, of course, the result of a long-continued 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, or the spinal cord, that in these cases the stimulating 

 changes are taking place (p. 188) in the central cells or efferent cells ' as well 

 as in the skin supplied by the afferent nerves. 



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

 that is, a frog prepared as described above, with the spinal cord separated 

 from the brain — 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 applied to the skin 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, 2 while the number of stimuli required 

 to produce a response remains practically constant. 



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 has 

 developed the idea of the summation of stimuli, implying at some part of the 

 pathway a piling up of the effects of the separately inefficient stimuli to a 

 point at which they ultimately become effective. 3 



The details of the changes involved in this summation and the place at 

 which the changes occur, are both obscure, but it would seem most probable 

 that summation is an expression of changes in the relations between the final 

 twigs of the afferent elements and the cell-bodies of the central or efferent 

 elements, which permit the better passage of the impulse from one element to 

 the other, for the evidence strongly indicates that the course of the impulse 

 can be interrupted at these junctions. 



The foregoing paragraphs have been concerned mainly with changes 

 occurring in the afferent portion- of the pathway. Next to be considered is 



1 Birge: Arehivfur Anatomic und Physiologie (Physiol. A.bthl>), 1882,8. 484. 



2 Ward : Ibid,, 1880. 



s Gad uud Goldscheider : " Ueber die Summation von Hautreizen," ZeU&chrift Jur klinische 

 Medicin, 1893, Bd. xx. Befte 4-ti. 



