THE CENTRAL NERVOUS SYSTEM. 



287 



various regions of the cord, it is evident that the areas which they 

 present vary with the plane of section : the accompanying diagram, 

 therefore, indicates their relative positions rather than their respective 

 extent. 



The framework of the spinal cord consists of the penetrating 

 pial processes, which divide the white matter into numerous areas 

 as well as convey the blood-vessels into the nervous tissue. From 

 the large fibrous partitions finer secondary trabeculze are given off ; 

 these, in turn, divide and subdivide until they become lost as delicate 

 fibrils among the nervous elements. In addition to the framework 

 of connective tissue contributed by the pia, the specialized sup- 

 porting tissue of the nervous system, the neuroglia, is distributed 

 throughout the cord, filling up the coarser meshes of the connective- 

 tissue reticulum and intimately uniting the more important nervous 

 elements. 



The neuroglia occurs immediately beneath the outer pial invest- 

 ment as a condensed peripheral zone, from which prolongations 

 accompany the pial septa, 



as well as intermingle with FIG. 320. 



the nerve-fibres ; among 

 the latter lie the charac- 

 teristic spider-cells, 

 sending their long, deli- 

 cate processes between 

 the fibres. 



The white matter of 

 the cord is made up seem- 

 ingly of great numbers of 

 small round nucleated ele- 

 ments, held together by 

 the supporting neuroglia. 

 These apparent cells are 

 the nerve - fibres in 

 transverse section, the 



supposed nuclei being really the cut axis-cylinders ; an irregularly 

 concentric striation is usually present around the axis-cylinder, this 

 appearance being produced by the partial distortion of the medullary 

 substance. The nerve-fibres of the cerebro-spinal axis possess no 

 neurilemma, the surrounding neuroglia affording the necessary 

 protection. 



The individual nerve-fibres composing the white matter of the 

 cord vary greatly in diameter (1-27 M) ; while the thick and the thin 

 fibres are found side by side in all regions of the cord, certain 

 columns are characterized by the predominance of thick fibres, 



Portion of white matter of human spinal cord : a, large 

 nerve-fibres in section ; 6, smaller fibres ; c , supporting 

 neuroglia ; d, spider-cell ; e, connective-tissue trabecula 

 containing blood-vessel, f ; g, spaces from which sections 

 of nerve-fibres have been lost.- 



