THE SPINAL CORD AND ITS NERVES 135 



Steiner, Philippson, Polimanti, Herrick and Coghill, and very 

 many others. 



Our most precise knowledge of the arrangement of the afferent 

 and efferent myelinated fibers in the spinal roots has been gained 

 by the application of Marchi's method (p. 48) to the study of 

 degenerations following accidental .and experimental injuries. 

 Nerve-fibers which have been cut off from their cells of origin 

 degenerate within about two weeks after the injury. It is, 

 therefore, possible by the microscopic study of a divided nerve 

 with Marchi's method (which stains only the degenerating mye- 

 linated fibers) to determine on which side of the injury are the 

 cells of origin from which these fibers arise. 



Figure 62 illustrates the effects of section of the spinal roots 

 made at four different places. In the first case section of the 

 mixed trunk peripherally of the union of the dorsal and ventral 

 roots is followed by degeneration of all of the myelinated fibers 

 of the nerve-trunk, showing that the cell bodies of all of these 

 fibers lie centrally of the injury. In the second case, section of 

 the ventral root close to the spinal cord is followed by degenera- 

 tion of all the fibers of this root without disturbance of those 

 of the dorsal root, showing that the ventral root fibers arise as 

 axons of cells within the spinal cord. In the third case section 

 of the dorsal root fibers peripherally of the ganglion and before 

 their union with those of the ventral root results in the degenera- 

 tion of all of the fibers of the mixed nerve which arise in the 

 spinal ganglion (sensory fibers), without loss of any motor fibers 

 from the ventral root. In the fourth case section of the dorsal 

 root on the central side of the ganglion is followed by degenera- 

 tion of all myelinated fibers of the central stump of this root, but 

 not of the peripheral part of the root or the spinal ganglion. 

 This shows that the cells of origin of these fibers lie in the spinal 

 ganglion and not, like those of the ventral root, within the spinal 

 cord. The peripheral processes of these ganglion cells, there- 

 fore, are dendrites, and the centrally directed processes which 

 compose the dorsal roots are axons (cf. Fig. 1, p. 25, and Fig. 

 56, p. 126). 



Another useful method for the solution of problems of this 

 character is the study of the fine structure of the cell bodies of 

 the neurons after such experimental lesions as those just des- 



