CENTRAL NERVOUS SYSTEM. 183 



we are led to anticipate in old age a correlation, on the one hand, between 

 the decrease in the quantity of functional substance in the cytoplasm, and a 

 decrease in the energy-producing power of the cells, and, on the other, 

 between the absorption of the cell-branches and a Limitation in the extenf to 

 which the neurones may influence one another. Both of these conditions are 

 characteristic of the nervous system during old age. 



B. The Nerve-impulse within a Single Neurone. 



The Nerve-impulse. — Neurones form the pathways along which nerve- 

 impulses travel. As introductory, therefore, to the study of the composite 

 pathways in the central system, comprising, as they do, several elements ar- 

 ranged in series, it becomes important to study the behavior of the nerve- 

 impulse within the limits of a single cell-element. 



Experimentally, the passage of the nerve-impulse is revealed by a wave of 

 change in the form of an electrical variation which passes along the nerve- 

 fibre in both directions from the point of stimulation. Under normal condi- 

 tions, the intensity of the electrical change does not vary in transit, though 

 for moderate electrical stimuli the strength of the electrical change ("action 

 current") is proportional to the strength of the stimulus. 1 It moves in the 

 peripheral nerves of the frog in the form of a wave some 18 millimeters in 

 length, at the mean rate of 30 meters per second, and this rate can be some- 

 what retarded by cooling the nerves and accelerated by warming them. In 

 mammals the rate in the peripheral nerves has been found by Helmholtz and 

 Baxt to be 34 meters per second, The nerve-impulse can be aroused at any 

 point on a nerve-fibre*provided a sufficient length of fibre be subjected to 

 stimulation. Mechanical, thermal, chemical, and electrical stimuli may be 

 used to arouse it, but just how the impulse thus started differs from that 

 normally passing along the fibres, as a consequence of changes in the cell- 

 bodies of which these fibres are outgrowths, is not known. It appears, how- 

 ever, that the impulses aroused by artificial stimuli are usually accompanied 

 by a much stronger electrical variation than accompanies the normal impulses. 



In the peripheral system the nerve-impulse, when once started within a 

 fibre or axone, is confined to that track and does not diffuse toother fibres 

 running parallel with it, although it does, of course, extend to all the branches 

 of the axone, whatever their distribution. 



The above-mentioned relations have been deduced from the study of die 

 peripheral nerves, and these morphologically are but parts of the axones, the 

 cell-bodies of which are located either in the central system proper or in the 

 spinal or sympathetic ganglia. 



The observations apply therefore to but one portion of the nerve-cell, and 

 our present purpose is to determine how far it is possible to apply them to 

 the entire nerve-cell, noting at the same time the modifications thus intro- 

 duced. 



Owing to the small size of nerve-cell bodies there arc. of course, very lew 

 '(Ireene: American Journal of Plti/siotoi/i/, 1NJIS, vol. i. p. 1 15. 



