624 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Points at which the Nerve-impulse can be Aroused. It appears pro- 

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



D ^ [ Excitation. \ 



I I l 1 Sec. | 



Excitation. 



\ I I I I I I I \ 18ec. \ 



FIG. 154. From a photographic record of the movements of the column of mercury in a capillary 

 electrometer (Gotch 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 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. 



a nerve-impulse, whether the stimulus be applied at the tips of the dendrons 

 or to the neuron in its course. 



There is, however, no indisputable evidence that within the central nervous 

 system the cell-bodies of nerve-cells can be made to discharge by the direct 

 application of electrical or other artificial stimuli to them, for there is no 

 locality suited for such isolated stimulation. In every place where such cell- 

 bodies are found they always lie more or less imbedded in the terminals of 

 neurons that have originated elsewhere, and hence present methods are not 

 fitted to decide whether the impulse is aroused in these cases indirectly by the 

 stimulation of the terminals or directly by the passage of the stimulus through 

 the cell-body alone. That artificial stimuli do in some way arouse the cell- 

 bodies to discharge is amply shown by the fact that when the cortex is stimu- 

 lated under the conditions just mentioned, the impulses continue to come from 

 the cortex after the stimulus itself has ceased to act. 



If after such a reaction the cortical layer containing the cell-bodies be cut 

 away, exposing the cut ends of the fibres which have originated from them, 

 and the stimulus be again applied, an impulse is to be detected in these fibres 

 so long as the stimulation is continued, but the impulses cease when the 

 stimulus stops. This difference in the time-relations and the form of the 

 impulses according to the presence or absence of the cortical layer is taken as 

 an indication that in the first instance the cell-bodies were stimulated, but it 

 still leaves the question of directness of the stimulation undecided. 



Probably every nerve-element in all its parts is to some degree irritable, 

 and the reports to the effect that the cell-bodies cannot be directly stimulated 

 are not supported by satisfactory proof that no nerve-impulses passed from the 

 point to which the stimulus was applied. 



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

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

 bility 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 that on stripping down the phrenic nerve by com- 



