618 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



changes in the outline of the nucleus are also to be noted, as well as the 

 decrease in their volume. The figures for the decrease in the volume of the 

 nucleus are given in the following table, showing the principal differences 

 observed on comparing the spinal ganglion-cells (first cervical ganglion) from 

 a child at birth with those from a man dying from old age at ninety-two years 

 (Hodge) : 



Child at birth ; male. Old man. 



Volume of nucleus 100 per cent. 64.2 per cent. 



Nucleoli visible 53 "" 5 " " 



Deep pigmentation " " 67 " " 



Slight pigmentation " " 33 " " 



Analogous changes were found by this investigator in the antennary gan- 

 glia of old honey-bees as compared with the corresponding ganglia taken from 

 those which had just emerged in the perfect form. These are also shown in 

 Figure 150. 



Since with the chemical and morphological variations which occur during the 

 entire growth-cycle there must go variations in the physiological powers, we 

 are led therefore 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 extent of the influ- 

 ence exercised by a given cell. Both of which defects are characteristic of the 

 nervous system during old age. 



B. THE NERVE-IMPULSE WITHIN A SINGLE NERVE-CELL. 



The Nerve-impulse. Nerve-cells 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 

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

 impulse within the limits of a single cell-element. 



Experimentally it is found that the nerve-impulse is revealed by a wave of 

 molecular change in the form of an electrical variation which passes along the 

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

 conditions the intensity of the electrical change does not vary in transit, but 

 it does change with changes in the strength of the initial stimulus. It moves 

 in the peripheral nerves of the frog in the form of a wave some 18 millime- 

 ters in length, at the mean rate of 30 meters per second, and this rate can be 

 somewhat retarded by cooling the nerves, and accelerated by warming them. 

 In mammals, the rate in the peripheral nerves has been found by Helm- 

 holtz 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- 



