274 



NORMAL HISTOLOGY. 



The immediate surface of the white matter, beneath the investing pia 

 mater, is formed by a tract of condensed neuroglia, the snbpial layer, that 

 is devoid of nervous elements and constitutes the definite outer boundary of 

 the cord. This zone consists of a dense interlacement of circular, longi- 

 tudinal and radial neuroglia fibrils, along with many glia cells. From the 

 deeper surface of this investment numerous bundles of fibrillae penetrate 

 between the nerve-fibres and become lost in the general supporting ground- 

 work. At certain points the bundles are replaced by robust septa which 

 imperfectly separate the nerve-fibres into groups or tracts, as conspicuously 

 exampled by the paramedian septum subdividing the posterior column. The 



Trabecula of 

 'glia 



i . J ' raoe 



, neurogh. 



Ill 



~*t2i 



Blood-vessel 

 in pia 



Subpial layer 

 of neuroglia 



Fir,. 320. Part of periphery of cord, showing subdivision of white matter by ingrowths of subpial layer 



of neuroglia. X 230. 



blood-vessels that enter the nervous substance of the cord from the pia mater, 

 accompanied by connective tissue, are surrounded by tubular sheaths of 

 neuroglia, and the same is true of the bundles of root-fibres of the spinal 

 nerves. With the exception of the connective tissue that enters with the 

 blood-vessels, the amount of mesodermic tissue included in the supporting 

 framework of the cord is inconsiderable, if, indeed, not wanting. 



The Fibre-Tracts. Although microscopical examination of ordinary 

 sections of the cord affords slight indication of a subdivision of the columns 

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

 combined evidence contributed by anatomical, pathological, embryological 

 and experimental investigation establishes the existence of a number of such 

 paths of conduction. With few exceptions, however, they are without sharp 



