122 THE PHYSIOLOGY OF NERVE 



because they again disappear in from three to eight days after the 

 injury and even if they are well protected by a tubular investment of 

 fascia. The degeneration, therefore, ceases with the formation of the 

 band fiber, a functionally inert strand of protoplasm. The central 

 stump, as has been stated above, degenerates in a typical manner only 

 for a distance of two or three nodes of Ranvier and hence, only those 

 segments are involved which have been directly exposed to the trauma. 



In addition, it has been noted that the cell-body of this neuron, 

 as well as such neurons as are in functional relation with it, undergo 

 certain changes which are arranged collectively under the name of 

 retrogressive degeneration. It is readily conceivable that an injury to 

 a chain of neurons must subject all of them to a certain inactivity which 

 is accompanied by a disturbance of their metabolism. The cell-body 

 becomes swollen and finally atrophies, this decrease in the quantity of 

 its cytoplasm being associated as a rule with an irregularity in the 

 contour of the nucleus and a change in its position to a place nearer 

 the surface of the cell. The chromatin material gradually disappears 

 so that the staining power of the cell becomes much diminished, 1 and 

 the more so, because this chromatolysis also affects the Nissl's granules. 

 It has also been shown by Dickinson 2 that many of these cells become 

 vacuolar and may indeed be completely destroyed, but these retro- 

 gressive changes require a relatively long time for their completion. 



The Morphological Changes of Regeneration. The regenerative 

 processes set in whenever the continuity of the neurons is reestablished, 

 provided, of course, that not too long a time has elapsed since the in- 

 jury. Thus, if a nerve is cut and its two ends are again brought into 

 contact immediately, the resulting changes are so fleeting that they 

 can scarcely be regarded as typifying Wallerian degeneration. Con- 

 currently, it may be assumed that a degeneration of long standing 

 can only be remedied by a regeneration occupying a correspondingly 

 long time. 



In accordance with the view that the neuron is not only the struc- 

 tural but also the functional unit of the nervous system, it is commonly 

 believed that the regeneration of the peripheral ends of the different 

 fibers can only be effected by outgrowths from the axis-cylinders of 

 the central stump (Ranvier). These neuroplasmic proliferations are 

 said to seek the old neurolemmal sheaths and to continue through them 

 into the end-organs. Opposed to this view is the one which holds 

 that a functional unit necessitates the presence of a number of neurons 

 arranged in series. In accordance with this conception, it is held by 

 Bethe and others that the neurolemma is composed of the remnants 

 of the neuroblasts from which the nerve-cells have originated. The 

 cutting of a nerve, therefore, would permit these elements to assume 

 their former characteristics and to give rise to regenerative changes in 

 the different fibers. While several facts might be mentioned in support 



1 Ranson, Jour, of Comp. Neur. and Psych., xvi, 1906, 265. 



2 Jour, of Anat. and Physiol., iii, 1869, 176. 



