490 ELEMENTARY PHYSIOLOGY 



the section, are known as descending degenerations, the 

 terms ascending and descending being thus used to denote 

 structural changes which start from the section and pass up 

 towards or down from the bi'ain respectively. Moreover, 

 since the parts which degenerate are limited as to their 

 transverse sectional area Avhile running for very consider- 

 able distances along the white matter of the spinal cord, 

 they ai'e usually spoken of as "tracts, "and since these 

 tracts serve very definitely for the transmission of 

 impulses up and down the cord, they denote very 

 definite paths of conduction along the spinal cord. 



Having thus explained the method of experimenting, 

 we may now state the chief results obtained by its 

 application. 



(A). Tracts of ascending degeneration, 



(i). The Posterior Column comprising^ the Postero- 

 median and Postero - lateral Tracts. — These tracts 

 occupy the posterior white columns of the spinal cord, 

 adjacent to the posterior fissure, in each half of the cord. 

 (Fig. 155, p.m., p.l.). 



The degenei-ation which marks out this tract follows not 

 only upon sections of the cord itself, but more especially 

 from cutting the posterior roots of the spinal nerves ; it is 

 therefore the result of a severance of the fibres in this 

 part of the cord from their nutritive centres in the cells of 

 the ganglion on the posterior root. Hence it marks the 

 course of fii)res passing up the cord from the posterior 

 roots, and denotes the path along which afferent (sensory) 

 impulses travel up from the spinal nerves. Since these 

 nerves are given oflf all along tlie cord, the tract is 

 necessarily found to exist throughout the whole extent of 

 the cord, and may be traced up into the spinal bulb, 

 where it ends on the side up which it passes. The 

 postero-median portion of the tract consists of fibres 

 coming from the lower part of the body, the postero- 

 lateral portion from the upper portion, hence the latter 

 only exists in the upper part of the cord . 



(ii). The Direct Cerebellar Tract. — This tract lies in 



