I038 HUMAN ANATOMY. 



true of the bundles of root-fibres of the spinal nerves. But apart from the connective tissue that 

 enters with the blood-vessels, the amount of mesoblastic tissue concerned in the supporting 

 framework of the cord is inconsiderable, according to some histologists, indeed, being 

 practically nothing. 



Fibre-Tracts of the White Matter. — Although microscopical examination 



of ordinary sections of the cord afiords slight indication of a subdivision of the 

 columns of white matter into areas corresponding with definite fibre-tracts, yet the 

 combined evidence of anatomical, pathological, embryological and experimental 

 investigation establishes the existence of a number of such paths of conduction. 

 With few^ exceptions, they are, however, without sharp boundaries and illy defined, 

 adjoining tracts often overlapping, and depend for their presence upon the fact that 

 nerve-fibres having the same function and destination proceed in company from the 

 same group of nerve-cells (nucleus) along a similar course. In addition to being pro- 

 vided with paths of conduction necessary for the performance of its function as a centre 

 for independent (reflex) impulses in response to external stimuli, the cord contains 

 tracts that connect it with the brain, as well as those that bring the various levels of 

 the cord itself into association. The white matter, therefore, contains three classes of ' 

 fibres : ( i ) those entering the cord from the periphery and other parts of the body ; 

 (2) those entering it from the brain ; and (3) those arising from the nerve-cells situated 

 within the cord itself. The first two constitute the exogc7ious, the last the e^idogenous 

 tracts. It is evident that some of these fibres constitute pathways for the transmission 

 of impulses from lower to higher levels and hence form ascending tracts, while others, 

 which conduct impulses in the opposite direction, form descending tracts. 



Since it is impossible to distinguish between these fibres by mere inspection of sections of 

 the adult normal cord, and, moreover, extremely difficult and practically impossible to follow 

 in such preparations the longer fibres throughout their course, advantage is taken of other 

 means by which differentiation of individual tracts is feasible. Such means include chiefly 

 the experimental and embryological methods. 



The experimental method depends upon the law discovered by Waller, more than half a 

 century ago, that when the continuity of a nerve-fibre is destroyed, either by a pathological 

 lesion or by the experimenter's knife, the portion of the nerve-fibre (the axone of a neurone) 

 beyond the break, and therefore isolated from the presiding nerve-cell, undergoes secondary 

 degeneration, while the portion remaining connected with the cell usually undergoes little or 

 no change. It should be pointed out, however, that occasionally the connected portion of the 

 fibre, and even the nerve-cell itself, undoubtedly exhibits changes known as retrograde degen- 

 eration, which, dependent upon the location of the lesion, may at times prove a source of 

 error in deducing conclusions. If a lateral section of one-half of the cord of a living animal be 

 made, and, after the expiration of from three to four weeks, transverse sections be cut and 

 appropriately prepared (by the methods of Marschi or of Weigert), certain groups of nerve- 

 fibres will present degenerative changes. It will be seen, however, that the degenerated tracts 

 in sections taken from above the lesion are not the same as those in sections from below the 

 division, showing that certain fibres have been involved in opposite directions, those arising 

 from nerve-cells lying below the lesion being affected with ascending degeneration, and those 

 from cells situated above with descending degeneration. In this manner, by careful study of 

 consecutive sections, much valuable information has been gained as to the origin, course, ter- 

 mination and function of many fibre-tracts within the central nervous system. 



The embryological method, also productive of important advances in our knowledge of 

 the nervous pathways, is based on the fact, first demonstrated by Meckel, that the nerve-fibres 

 of the central nervous system do not all acquire their medullary sheath at the same time. 

 Taking advantage of such variation, as suggested by Meynert and later extensively carried out 

 by Flechsig and others, upon staining sections of embryonal tissue with reagents that color 

 especially the mednllary substance, it is possible to differentiate and follow certain fibi^e-tracts 

 in the foetal cord with great clearness, since only those tracts are stained in which the myelin is 

 already formed. It is of interest to note that, in a general way, the order in which the different 

 strands of the cord acquire their medullary coat accords with the sequence in which nervous 

 function is assumed by the foetus and child. Thus, the paths required for spinal reflexes (the 

 posterior and anterior root-fibres) are first to become medullated (fourth and fifth fcetal 

 months); those bringing into association the different segments of the cord next (from the 

 fifth to the seventh month) acquire myelin ; those connecting the cord with the cerebellum 

 follow somewhat later, while those establishing relations with the cerebral cortex are last and 

 do not begin to medullate until shortly before birth. 



