120 



THE PHYSIOLOGY OF NERVE 



goes merely a gradual retrogression. The peculiar distribution of 

 these fibers also permits of a third cut being made, namely, at a point 

 distally to the spinal ganglion. In the latter case, the degeneration 

 involves the distal axons, leaving the entire posterior root intact until 

 subsequently affected by retrogressive changes (Fig. 67, III). 



Very similar conditions prevail inside the central nervous system. 

 Thus, it may be noted that the anterior pyramidal tracts of the spinal 

 cord are formed by axons derived from cells in the motor area of the 

 cerebral cortex, whereas the posterior columns are made up of axons. 



PR 

 Fig. 67. 



Fig. 67. — The Course op the Degeneration in the Roots of the Spinal Cobd. 

 AR, anterior root; PR, posterior root; /, division of anterior root; II, division of 

 posterior root centrally to ganglion; ///, division of posterior root distally to spinal 

 ganglion. The degenerated fibers are indicated in black. 



Fig. 68. — Diagram to Illustrate the Direction of Degeneration in Spinal 

 Neurons. 



The degenerated portion is indicated by dotted lines. 



the cell-bodies of which lie either in the spinal ganglion or at a low 

 level of the cord. The former are motor and the latter sensory in 

 their function. Consequently, a division of the spinal cord, say, at 

 the level of the first thoracic vertebra must be followed by an outward 

 degeneration of the pyramidal tracts and an inward degeneration of 

 the posterior columns. The former is generally called descending 

 and the latter ascending degeneration. 



The Morphological Changes of Degeneration. — The foregoing 

 discussion must have shown that the cell-body is the nutritive center 



