CENTRAL NERVOUS SYSTEM. 189 



was also made to determine the rhythmic character of the negative varia- 

 tions 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 electrom- 

 eter. This result suggests that the impulses sent out from the spinal cord 

 by the normal discharge of the motor nerve-cells may differ from the 

 impulses artificially aroused in the lesser intensity of the electrical changes 

 that accompany them. 



Rate of Discharge. — The rate at which the nerve-cells discharge, as 

 shown by the number of impulses which produce tetanus of a muscle indi- 

 rectly excited, either by artificial stimulation of the nerve-elements in animals 

 or by voluntary impulses in man, is about ten impulses 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 sonic 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 



Points at which the Nerve-impulse can be Aroused. — It is probable 

 that the excitation of any part of a nerve-cell is capable of producing a nerve- 



D < w*. I Excitation. | 



I 1 Sec. | 



Excitation. 



I I i I I I I I I I Sec- I 



Fig. 79. — From a photographic record of the movements of the column of mercury in a capillary 

 electrometer (Uotch and Horsley). The arrow shows the direction in which the record is to be read 

 The upper curve {D) shows the period of excitation by the interrupted current; this is followed by a 

 series of waves in the record showing a number of separate impulses sent down from the cortex after 

 electrical stimulation has ceased. In the lower curve (C), the exciting electrodes were applied to the 

 white matter directly, the cortex having been removed. The record shows that in this case no impulses 

 pass after the stimulation has ceased. 



impulse, whether the stimulus be applied at the tips of the dendrites or t<> 

 the axone in its course. 



Irritability and Conductivity. — In general, parts of the system which 

 are irritable are also conductive, but there are special cases in which the 

 irritability of the nerve-fibre can be distinctly separated from its conductivity, 

 the latter being present while the former is absent. 



It is an old observation thai on stripping down the phrenic nerve by 

 compressing it between the thumb and forefinger and sliding these along the 

 nerve, a contraction of the diaphragm is caused. The part of the nerve thus 

 stimulated is soon exhausted. If, now, the same operation is repeated on a 

 portion of the nerve lying nearer the spinal cord, contraction of the diaphragm 

 again follows. This result was originally used to support the theory of a 

 'Schliferand Horsley: Journal of Physiology, 1885, vol. vii.; Schafer, Ibid. 



