628 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



On analyzing the condition thus established by an amputation it is seen 

 that the cells located in the spinal cord are deprived by such an operation of 

 one principal group of incoming impulses, namely those which arrive through 

 the dorsal root-fibres that are most closely associated with them ; but at the 

 same time there remain many other ways in which these same cells are nor- 

 mally stimulated. The efferent pathway from these cells is incomplete, and 

 the impulses which must pass along the stumps are inefficient. That im- 

 pulses do pass along the stumps of the efferent roots is beyond question, since, 

 when the distal portion of an efferent nerve is cut off the cell can be shown 

 to still discharge through the portion of the fibres connected with the cell- 

 bodies. Moreover, the muscles of any stump tend to execute the associated 

 contraction which they normally perform, thus showing that the group of 

 cells is fully innervated, although its discharge is without mechanical signifi- 

 cance, and finally there is always a tendency to the regeneration of the cut 

 fibre which indicates activity through its entire length. 



It is therefore not improbable that after amputation impulses do pass down 

 even those fibres which end without physiological connections. It is explica- 

 ble from this that in the case named the spinal ganglion cells should be more 

 affected than those of the spinal cord. Further, since the efferent cells of the 

 leg are more commonly innervated bilaterally than are those of the arm, we 

 might expect the efferent cells in the cervical region to be more readily affected 

 by an amputation. 



Wherever in the central system a group of fibres forms the chief pathway 

 for the impulses arriving at a given group of cells, then the destruction of 

 these afferent fibres brings about the more or less complete atrophy of the 

 cells with which they are secondarily associated, and this effect is the more 

 marked the younger the animal at the time of injury. Examples of this 

 relation are found in the " nuclei " of the sensory cranial nerves. 



Thus the activity of a given cell has the value of contributing to the 

 strength of its own nutritive processes, and different cell-elements, so far as 

 they are physiologically united, stand in a nutritive or trophic relation to one 

 another such that the cell receiving impulses is in some measure dependent for 

 its nutrition on the cell which delivers the impulses to it. 



Fatigue. It is a familiar fact that living tissues may be fatigued. In 

 the nervous system the signs of fatigue are both physiological and histological, 

 but it is to the latter changes only that attention will be here directed. 



Not only is the food-supply to the nerve-cells, as represented by the quality 

 and quantity of the plasma, variable, but the cells themselves are subject to 

 wide variations in their power to use the surrounding substances. 



When in a nerve-trunk containing both afferent and efferent spinal root- 

 fibres passing to a limb, the afferent fibres are stimulated by a farad ic current 

 applied intermittently, changes in the cell-bodies in the spinal ganglion are to 

 be observed (Hodge). 



When this experiment is made on a cat, and, after death, the sections 

 from the stimulated are compared with those from the corresponding but 



