THE CENTRAL NERVOUS SYSTEM. 



and (2) by studying the degeneration which follows various lesions, 

 either pathological or experimentally produced. 



The Myelination Method. This method depends upon the fact 

 that in the course of development the various conducting tracts acquire 

 their myelin sheaths at different periods, so that by examining embryos 

 at different stages of development it is possible to determine the limits 

 of each particular tract. The longer fibres in the spinal cord become 

 myelinated later than those fibres which run a shorter course ; thus the 

 pyramidal tracts do not acquire their myelin till after birth (fig. 9). 

 The appearance of function coincides 

 with the period of acquisition of 

 myelin. 



The Degeneration Method. The 

 degeneration method is based upon the ^ 

 fact that when a medullated nerve fibre 

 is divided, the portion which is cut off *? 

 from the nerve cell of which it forms r /^...j 

 a part undergoes degeneration. If the 

 spinal cord is cut across in an animal, 

 certain tracts degenerate in the part 

 above the section, and other tracts de- 

 generate in the part below the section. 

 The former are said to have under- 

 gone ascending degeneration, and their _ 



FIG. 9. Section through the cervical 



fibres are axons of cells which lie below spinal cord of a new-born child, 



stained by Weigert's method to 

 show absence of medullation in 

 rramidal tract. (Bechterew. 



pyramidal tract ; Fe, direct cerebellar 

 tract ; rp', posterior root fibres. 



the point of section. The latter are 



said to have undergone descending de- py 



generation, and their fibres are axons p^ w f ar f ng ' S Prindples f 



Of cells which lie above the point of ca> an^il ^commissure; Fp, crossed 



section. The extent of the degenera- 

 tion varies in different parts of the 

 white substance. In the case of the' short tracts it extends for a 

 limited distance from the section; in the case of the long tracts it 

 may extend from the section to the upper or lower extremity of 

 the spinal cord. From what has been said of the function of the 

 neuron, it will be evident that the tracts which show descending de- 

 generation are those which convey descending impulses, while those 

 which show ascending degeneration are those which are concerned with 

 conducting ascending impulses. 



The chief conducting tracts of the spinal cord are shown in 

 fig. 10. 



