CENTRAL NERVOUS SYSTEM. 197 



way from these cells is incomplete, and the impulses which must pass along 

 the stumps are inefficient. That impulses do pass along the stumps of the 

 efferent roots is beyond question, since, when the distal portion of an effer- 

 ent nerve is cut off, the cell can be shown to still discharge through the por- 

 tion of the fibres connected with the cell-bodies, and, finally, there is always 

 a tendency for the cut fibre to regenerate, which indicates activity through 

 its entire length. 



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 about which they terminate, and this effect is the more marked the 

 younger the animal at the time of injury. Examples of this relation are 

 found in the behavior, of the nuclei of the sensory cranial nerves. 



Thus the activity of a given cell contributes to the strength of its own 

 nutritive processes, and different cell-elements, so far as they are physiologi- 

 cally associated, stand in a nutritive or trophic relation to one another such 

 that the receiving cell is in some measure dependent for its nutrition on the 

 cell which stimulates it. 



Degeneration of Nerve-elements. — -All parts of a nerve-cell are under 

 the control of that portion of the cell-body which contains the nucleus ; in 

 this respect the nerve-elements are similar to other cells which have been 

 studied, and in which the nucleated portion of the cell is found to be alone 

 capable of further growth. It was shown by Waller 1 that when sepa- 

 rated from the cell-body of which it w r as an outgrowth, a nerve-fibre belong- 

 ing to the peripheral nerve soon degenerate from the point of section to its 

 final distribution. The process is designated as secondary or " Wallerian 

 degeneration." According to recent studies on this subject, 2 this degener- 

 ative change occurs practically simultaneously along the entire length of the 

 portion cut off. The changes following the section of medullated nerve- 

 fibres consist in a fragmentation of the axis-cylinder followed by its dis- 

 appearance ; enlargement and multiplication of the nuclei of the medullary 

 sheath, and absorption of the medullary substance, so that in the course of 

 the fibres there remains at the completion of the process the primitive sheaths 

 together with the sheath-nuclei. In the early stages of this process the 

 medullary sheath, moreover, undergoes some changes, the result of which is 

 that it stains more deeply with osmic acid, and hence appears very black in 

 comparison with the normal fibres about it (Marchi). These changes, as 

 shown by the method of Marchi, may follow even slight injuries to the nerve- 

 fibres — such as compression for a short time. 



Concerning the progress of degenerative changes in the non-mcdiillatcd 

 fibres information is scanty. Bowditch and Warren 3 observed that when 

 the sciatic nerve of the cat was sectioned, degeneration of the motor and 



1 Nouvelle methode anatomi<]m pour ['investigation du Syateme nerveux, Bonn, 1851. 

 'Howell and Huber: Journal of Physiology, 1S92, vol. xii. 

 ' Journal of Physiology, 1885, vol. vii. 



