132 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



following stimuli is not possible in the case of the nerve fiber. 

 In the nerve cell, on the contrary, it is usually taught that the 

 power of summation is a characteristic property, although it 

 may be said that the proofs for this belief are not very direct. 

 It is pointed out that, while a single stimulus applied to a sensory 

 nerve may be ineffective in producing a reflex response from the 

 central nervous system, a series of such stimuli will call forth a 

 reaction. In this case it is assumed that the effects of the suc- 

 ceeding stimuli are summated within the nerve cells through which 

 the reflex takes place, and, generally speaking, it is assumed in 

 physiology that the nerve centers are adapted by their power of 

 summation to respond to a series of stimuli or to continuous stimu- 

 lation. The best examples of this kind of action are obtained 

 perhaps from sensory nerves, in which case we judge of the intensity 

 of the cell activity by the concomitant sensation, or by a reflex 

 response. 



Response of the Nerve Cell to Varying Rates of Stimula- 

 tion. The various parts of the neuromuscular apparatus 

 namely, the nerve cell, the nerve fiber, and the muscle fiber have 

 different degrees of responsiveness to repeated stimuli, and this 

 responsiveness varies, moreover, for the different kinds of mus- 

 cles and of nerve fibers, and, probably for the different kinds 

 of nerve cells. The motor cells of the brain discharge their 

 motor impulses normally at a rate probably of about 10 per second 

 (see p. 44), and it is very interesting to find that, if these cells 

 are stimulated artificially, their rate of motor discharge does not 

 keep pace with that of the stimulation employed, but occurs at 

 about the same rate as the normal, namely, at about 10 per 

 second. Thus, Horsley and Schafer* found that in monkeys, dogs, 

 cats, and rabbits, stimulation of the motor regions of the cortex 

 or the motor cells in the cord gave tetanic muscular contractions, 

 which from their graphic records were evidently composed of 

 simple contractions following at an average rate of 10 per second, 

 although the stimuli applied to the center might vary in rate from 

 10 to 50 per second. Similar results by a somewhat different 

 method were obtained by Broca and Richet.f These authors point 

 out, moreover, that no mental act can be repeated more rapidly 

 (on the average) than 10 times per second. If one, for instance, 

 attempts to think a series of syllables or words in a given phrase 

 the maximum of rapidity with which each syllable can be clearly 

 thought is at the rate named. The authors last named believe, 

 therefore, that the minimal duration for an intellectual act is 

 probably approximately about y-g- of a second. These facts, so 



* Horsley and Schafer, " Journal of Physiology," 7, 96, 1886. 



t Broca and Richet, "Journal de physiol. norm, et pathol," 1897, p. 864. 



