188 



AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



the region where ftie efferent cell-bodies are 

 clustered together, that not only were impulses 

 sent out of the cord, causing the muscles to 

 contract, but they continued to be sent out for 

 some seconds after the injury. 



When an electrical stimulus is applied to the 

 cerebral cortex so as to stimulate the cells there 

 present, the discharging cells may also continue 

 to send out impulses for some time after the 

 cessation of the stimulus. 



Experiments showing the multiple character 

 of the impulses aroused within the central 

 system have been made by Gotch and Horsley. 1 



When the motor cortex of a monkey was 



stimulated (Fig. 78) by means of the faradic 



current, the muscles which by this means were 



made to respond showed a long tonic contraction 



followed by a series of shorter clonic ones (Fig. 



79, D). When the spinal cord had been cut 



across, the cortex was again stimulated, and the 



electrical changes in the fibres of the cord which 



convey the impulses from the cortex to the 



spinal centres were investigated by means of the 



capillary electrometer. By this means a curve 



(Fig. 79, D) was obtained as a record of the 



negative variations passing along these fibres. 



This latter curve corresponds with the record 



for the muscular contraction, and hence the 



relation between the two series of events is 



evident. It appears, therefore, that the cortical 



cells after the cessa- 

 -Mercury. 



^Sulphuric acid 10%. 



Microscope. 



nj 



Miriiiri/. 



tion of the stimulus 

 still continue to dis- 

 charge in a rhyth- 

 mical manner. When 

 the cortex had been 

 removed, and the 

 electrodes were ap- 

 plied directly to the 

 underlying fibres, the 

 discharge of the im- 

 pulses was found to 

 cease with the stoppage of the stimulus. The presence of the cortex was 

 therefore necessary for the continued discharge (Fig. 79, C). The attempt 



1 Proceedings of the Royal Society, London, 1888. 



Fig. 78.— Schema illustrating the experiment fur determining the num- 

 ber of separate nerve-impulses passing down the spinal cord upon stimula- 

 tion of the cortex (from experiments on the monkey ; Horsley) : E, E. elec- 

 trodes, intended to be on the "leg area." Where the cord is interrupted, 

 one non-polarizable electrode is placed over the cut end of the pyramidal 

 fibres going to the lumbar enlargement ; the other, on the side of the cord. 

 These lead to the capillary electrometer, in which the column of mercury 

 moves each time an impulse passes. 



