CENTRAL NERVOUS SYSTEM. 



627 



of chemical changes due to activity and to age are very evident. The nature 

 of these latter changes is quite unknown. There is general consensus that 

 the alkalinity of the nerve-tissues is decreased during activity, and this decrease 

 in alkalinity may amount at times to a positively acid reaction. 1 This change, 

 too, is better supported by the observations made where the cell-bodies are 

 numerous, than by those made where the fibres are alone present. 



Trophic Influences. When a nerve-cell is not kept active by the passage 

 of nerve-impulses through it, it usually atrophies and may degenerate. The 

 reason for this appears to lie in the fact that the loss of those changes 

 which accompany the nerve-impulses decreases the vigor of the nutritive 

 exchange with the result of causing a steady diminution in the volume of the 

 cell or even its disintegration. Such changes are found, for instance, in the 

 nerves after the amputation of the limb to which they were supplied. 2 



The result of an amputation is that portions of the neurons originating from 

 cell-bodies located either in the ventral horns of the spinal cord, or in the 

 cells of the spinal ganglion, are removed. In the latter case the normal 

 pathway for the incoming impulses is interrupted at its peripheral end, and in 

 the former the last part of the pathway by which the impulse is delivered at 

 the periphery is destroyed (see Fig. 156). 



FIG. 156. Cross section of the spinal cord of the chick, X 100 diameters (van Gehuchten) : D, dorsal 

 surface ; V, ventral surface ; d. r, dorsal root ; v. r, ventral root ; g, spinal ganglion. On the left the arrows 

 indicate the direction of the larger number of impulses in the dorsal and ventral roots respectively. The 

 small arrow on the right dorsal root calls attention to the fact that some neurons arising in the ventral 

 plate emerge through the dorsal root and convey impulses in the direction indicated. 



The disturbance caused in the two sets of cells is, however, not the same. 

 In the case of the cells of the spinal ganglion the chief pathway by which they 

 are stimulated under normal conditions, is so far mutilated that only a com- 

 paratively small number of impulses passes over them. That some do pass, 

 is indicated by the sensations apparently coming from -the lost limbs sensa- 

 tions which are often very vivid and minutely localized. 3 



1 Gscheidlen : Archiv fur die gesammte Physiologic, 1874, Bd. viii. 



2 Grigoriew : Zeitschrift fur Heilkunde, 1894, Bd. xv. 



3 Weir-Mitchell : Injuries of Nerves, Philadelphia, 1872. 



