CENTRAL NERVOUS SYSTEM. 



623 



again stimulated and the 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. 154, 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 cessation of the 

 stimulus still continue to discharge in a rhyth- 

 mical manner. The attempt was also made 

 to determine the rhythmic character of the 

 negative variations in the motor nerve-trunk 

 between the cord and the contracting muscle, 

 but the changes there present, though sufficient 

 to cause contractions of the muscle, were not 

 strong enough to be recorded by a delicate 

 capillary electrometer. This result suggests 

 that the impulses sent out from the spinal cord 

 by the normal discharge of the motor nerve- 

 cells in it may differ from the impulses artifi- 

 cially aroused in the lesser intensity of the 

 electrical changes that accompany them. The 

 rate at which the nerve-cells discharge, as 

 shown by the number of impulses which pro- 

 duce tetanus of a muscle indirectly excited, 

 either by artificial stimulation of the nerve- 

 elements in animals or by voluntary impulses 



in man, is about 

 ten per second. It 

 appears that at least 

 the cortical cells 

 and those of the 

 spinal cord have 

 the same rate of 

 discharge, and that 

 this rate is the same 

 in some mammals 

 (dogs, cats, rabbits, 

 and monkeys) as in 



man. Hence a tendency to discharge about ten times a second may be assumed 

 .as characteristic of the mammalian nerve-cell. 1 



1 Scliafer and Horsley : Journal of Physiology, 1885, vol. vii. Schafer : Ibid. 



Mercury. 



, "Sulphuric acid 10%. 



Microscope. 



Mercury. 



FIG. 153. Schema illustrating the experiment for 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. 



