THE NEURON AND ITS CONDUCTING PATHS 119 



latter functionally useless. This enforced inactivity causes the cell- 

 body to lose its irritability and to undergo very characteristic morpho- 

 logical changes which present themselves as an initial turgescence and 

 final atrophy of its cytoplasm and nuclear material. The Nissl's gran- 

 ules become indistinct and finally disappear so that the cytoplasm 

 assumes a more homogeneous character. It is to be emphasized, 

 therefore, that the degeneration begins at the seat of the trauma and 

 advances from here in a peripheral as well as in a central direction. 

 It involves first of all the entire distal end of the nerve and later on 

 also its central stump, inclusive of the corresponding cell-bodies and 

 their dendrites. The degeneration progressing in a central direction, 

 is commonly designated as retrogressive degeneration. Lastly, it is to 

 be noted that these retrogressive changes do not stop at the next 

 synapse, but also implicate those neighboring neurons which are in 

 functional relation with the neuron primarily affected by the injury. 

 The cause of this retrogression must again be sought in the Inactivity 

 forced upon the correlating neurons by the trauma to one of their 

 series This type of degeneration may be characterized as tertiary, 

 because it is not the direct result of the lesion, but develops only in 

 the course of time in those neurons which formerly acted in harmony 

 with the injured neuron. 



We have seen that neurons are arranged in such a manner that 

 their axons conduct either in an efferent or afferent direction. Inas- 

 much as the degeneration first involves that segment of the fiber which 

 has been disconnected from the cell-body, the morphological changes 

 must advance along an efferent fiber in a direction from the center 

 toward the periphery. In an afferent fiber conditions are not so 

 simple. The cell-body is situated in between its processes. The de- 

 generation, therefore, may affect either its distal or its central proc- 

 esses. This statement will be more easily understood if a brief refer- 

 ence is made at this time to the so-called Wallerian law of degeneration. 

 It is a well-known fact that the anterior roots of the spinal cord are 

 formed by axons which are derived from large ganglion cells situated 

 in the corresponding horn of the gray matter. These axons, therefore, 

 conduct toward the periphery and are wholly efferent or motor in 

 their function. For this reason, a division of this root must be followed 

 by a degeneration which progresses outward from the level of the cut 

 until all the terminals have become involved (Fig. 67, I). The central 

 stump of this root as well as the corresponding cell-bodies and their 

 dendrites, will be affected in the course of time by retrogressive degen- 

 eration. The posterior root of the spinal cord, on the other hand, is 

 made up of axons which arise in cells situated in the so-called spinal 

 ganglia. Their function is afferent or sensory, and hence, their direc- 

 tion of conduction is from the periphery to the center. This fact 

 implies that the division of this root must give rise to a degeneration 

 involving the end still connected with the cord (Fig. 67, II), whereas 

 its other end which has remained in contact with the ganglion, under- 



