THE SPINAL CORD. 



MICROSCOPIC STRUCTURE OF THE SPINAL CORD. 



The white substance of the spinal cord is almost wholly composed of longitudinally 

 coursing medullated nerve-fibres, which in carmine-stained transverse sections of 

 the cord (fig. 12) appear as clear rings with a stained dot the section of the axis- 

 cylinder either in the middle of the ring or shifted somewhat to one side. The 

 fibres vary much in size, and in many parts of the section larger and smaller fibres 

 are intermixed, but some parts are characterised by containing many large fibres, 

 others for the most part small fibres. The largest fibres are in the circumferential 

 part of the anterior and lateral columns (and especially in the direct cerebellar tract), 

 the smallest in the part of the lateral column in the neighbourhood of the processus 

 reticularis, in the marginal bundle of Lissauer near the apex of the posterior horn, and 

 in the postero-mesial column. Very small fibres also occur scattered over the white 



Fig. 12. A SMALL PORTION OF A TRANSVEKS3 SECTION OP THE WHITE MATTER OF THE EUMAN SPINAL CORD. 



HIGHLY MAGNIFIED. (E. A. S.) 



a, a, superficial neuroglia ; b, b, transverse section of part of the lateral column of the cord (direct 

 cerebellar tract). The dark points are the axis-cylinders, and the clear areas the medullary substance 

 of the nerve-fibres : the superficial neuroglia exhibits the appearance of a fine network in which only the 

 nuclei of the neuroglia-cells are seen. One or two corpora amylacea (c.a) are embedded in the neuroglia, 

 which extends inwards among the nerve-fibres. 



substance, especially in the anterior column. The white columns are imperfectly 

 divided into secondary columns by incomplete septa of fibrillar connective tissue 

 which are prolonged inwards from the inner layer of the pia mater, and convey 

 blood-vessels to the interior of the cord. 



Immediately beneath the pia mater and closely investing the cord externally is a 

 layer of what in the fresh condition appears a homogeneous substance with nuclei 

 embedded in it here and there. In sections of the cord hardened in alcohol or 

 chromic salts, the substance in question is finely reticulated (fig. 12, a, a). The layer 

 which it forms is very thin over some parts of the surface but comparatively thick 

 in others, and where the pia septa pass into the cord, it accompanies and invests 

 them and their ramifications in the white substance, passing with them between the 

 irregular bundles of nerve-fibres. Not only does this subpial reticular substance 

 accompany the prolongations of the fibrillar tissue and largely assist in forming the 

 incomplete septa above mentioned, but it extends independently amongst the indi- 

 vidual nerve-fibres, occupying the interstices between them, and serving as a uniting 

 medium in which they are embedded. Hence it was named by Virchow the 

 neuroglia (nerve-cement). The nuclei in it belong for the most part to branched 

 fibrillated cells (neuroglia-cells) which occur in considerable numbers and may be 



