WHITE MATTER OF THE SPINAL CORD. 



of the cord, appears to be composed of innumerable, closely set, small cells, held 

 together by delicate supporting tissue. These apparent cells are the medullated 

 nerve-fibres cut transversely, in which the sectioned axis-cylinders show as deeply 

 stained dots, that commonly lie somewhat eccentrically and are surrounded by deli- 

 cate irregularly annular striations representing the framework of the medullary coat. 

 The nerve-fibres of the cerebro-spinal axis are without neurilemma, the lack of this 

 sheath being compensated by a slight condensation of the neuroglia around the 

 fibres. Seen in transverse sections this investment appears as the ring that gives 

 a definite outline to the fibre. 



The individual nerve-fibres vary greatly in size, even within the same tract large and small 

 fibres often lying side by side. The smallest may be less than .005 mm. and the largest over 

 .025 mm. In a general way, the diameter of the fibre bears a direct relation to its length, those 



Fig. 892. 



Trabecula of neuroglia 



Neuroglia cell 



Nerve-fibre 



Blood-vessel in pia 



Subpial layer 

 of neuroglia 



Peripheral part of transverse section of spinal cord, showing nerve-fibres subdivided into groups by ingrowth of 



subpial layer of neuroglia. X 230. 



having an extended course being larger than shorter ones ; it follows that the fibres occupying 

 the peripheral parts of the white matter, particularly in the lateral columns, are more frequently 

 of large diameter than those near the gray matter. 



The immediate surface of the white substance beneath the pia mater is formed by a con- 

 densed tract of neuroglia, the subpial layer, from .O2O-.O4O mm. in thickness, that is devoid of 

 nervous elements and forms the definite outer boundary of the cord. This zone consists of a 

 dense interlacement of circular, longitudinal and radial neuroglia fibrils among which numer- 

 ous glia cells are embedded. From the deeper, surface of this ensheathing layer numerous 

 bundles of fibrilke penetrate between the subjacent nerve-fibres to become lost in the general 

 supporting ground-work. At certain places the bundles are replaced by robust septa by which 

 the nerve-fibres are imperfectly divided into groups or tracts, as conspicuously seen in the pos- 

 terior column where the paramedian septum effects an imperfect subdivision into the tract of 

 Goll and of Burdach. The blood-vessels that enter the nervous substance from the pia, accom- 

 panied by connective tissue, are surrounded by tubular sheaths of neuroglia, and the same is 



