838 THE NERVE SYSTEM 



a sheath of neuroglia immediately beneath the pia. Numerous septa, derived from 

 the pia, but always coated by a thin layer of neuroglia, pass into the white sub- 

 stance to separate the respective bundles of fibres and are often interwoven between 

 individual nerve fibres, acting as a supporting framework in which they are em- 

 bedded. In addition to the longitudinal fibres there are shorter and less numerous 

 transverse fibres collected into the so-called commissure bundle or white commissure . 



The longitudinal fibres constitute the conducting tracts. Although a purelv 

 anatomical examination fails to reveal the functional relations in these fibre 

 bundles, the structural alterations which ensue (in accordance with the laws 

 of Waller) in the distal portion of a neurone whose proximal portion has been 

 destroyed, the progressive myelinization of separate tracts in the embryo and 

 infant (as proved by the researches of Meynert and Flechsig), the comparative 

 anatomy method and electrophysiological experimentation have rendered possible 

 the demonstration of the origin and destination of the various conducting systems 

 or tracts with almost mathematical accuracy. While some fibres pursue a lengthy 

 course, serving as conduction paths between the brain centres and the various 

 spinal centres, others are shorter and serve to associate different spinal levels 

 in juxtaposition or relatively not far distant. It must be borne in mind that the 

 gray substance, intercalated as it is in the course of the impulses which pass to and 

 fro in the cord, contains the neural elements which are either (a) the source 

 or (6) the destination of these impulses, and thus complete the nerve cycle requisite 

 for the organization of the functions belonging to the cord. The motor and 

 sensor phenomena, though interacting, depend upon distinct nerve elements 

 which, because of their functional relationship, or because of the direction in 

 which they convey impulses, are generally referred to as motor or efferent and sen- 

 sor or afferent neurones; and in the spinal cord usage has sanctioned the employ- 

 ment of the terms descending and ascending for tracts conveying motor and sensor 

 impulses, respectively. Anatomically speaking, however, it is preferable to de- 

 scribe the tracts with reference to their origins and termini (as cerebrospinal, 

 spinothalamic, etc.), when known, or to their topographical relations as studied 

 in trans-sections of the cord. 



For the purposes of description it is convenient to classify the longitudinal 

 fibres into three general systems: (1) The cerebrospinal system of axones forming 

 conduction paths for efferent impulses from the cerebrum to the spinal centres 

 for peripheral organs, and axones for afferent impulses received in the spinal 

 centres from the periphery and conveyed in turn to the cerebrum. (2) The 

 spinocerebellar system, consisting of conduction paths, afferent and efferent between 

 the cerebellum and the spinal centres. (3) Numberless association systems 

 strictly confined within the cord (or only extending into the medulla oblongata), 

 composed of shorter or longer axones which serve to associate not only different 

 levels of the same spinal segment, but also the different segments that are in juxta- 

 position or more remotely situated. These spinal association axones form 

 bundles in close contact with the central gray column and are termed the fasciculi 

 proprii or ground bundles. 



Tracts of the Spinal Cord. In the dorsal column there are the following tracts: 



Ascending : 



1. Fasciculus gracilis (Golli). 



2. Fasciculus cuneatus (Burdachi). 



Descending: |-^Y\AftJt. ^ 



1. Comma tract (Schultze). V '% ^ 



' VDV NX 



2. Median oval tract (Flechsig).J 



Associating : ' -y ' V- ' o & v S 



1. Fasciculus dorsal is proprius. V f\S %* (L 



2. Dorsal cornucommissural tract.? 



3. Septomarginal tract (Bruce). / 



