218 



THE ESSENTIALS OF HISTOLOGY. 



These are, however, united by an isthmus or bridge, which is composed 

 anteriorly of transversely crossing white fibres (white commissure), 

 posteriorly of grey matter (grey commissure), in the middle of which is 

 a minute canal lined by ciliated epithelium (central canal, fig. 251, c.c., 

 and 255). 



Each lateral half of the spinal cord contains a crescent of grey 

 matter, which is joined to the corresponding crescent of the opposite 

 side by the grey commissure. Of the two cornua of the crescent the 

 posterior is the narrower and comes near the surface of the cord; 

 opposite to it the bundles of the posterior nerve-roots enter the cord. 

 The bundles of the anterior nerve-roots enter the anterior cornu. 



The white matter of each half of the cord is subdivided by the passage 

 of the nerve-roots into the cornua into three principal columns 

 anterior, lateral, and posterior. In the upper part of the cord the 



FlG. 251. A SMALL PORTION OF A TRANSVEKSE SECTION OP THE HUMAN SPINAL CORD 



IN THE REGION OF THE LATERAL COLUMN, TO SHOW THE SUPERFICIAL NEUROGLIA. 

 a, a, superficial neuroglia ; 6, b, transverse section of part of the lateral column of the cord, in 

 which the dark points are the axis-cylinders, and the clear areas the medullary substance 

 of the nerve-fibres. The superficial neuroglia is seen to exhibit the appearance of a fine 

 network in which numerous nuclei and one or two corpora amylacea, are embedded, and 

 to extend inwards among the nerve-fibres. 



posterior column is subdivided by a septum of connective tissue into 

 two the postero-mesial column or funiculus gracilis, and the postero-lateral 

 column or funiculus cuneatus. The white matter is composed of longi- 

 tudinally coursing medullated nerve-fibres, which in sections stained 

 with carmine or aniline blue-black appear as clear circular areas with a 

 stained dot, the axis-cylinder, near the middle (fig. 251); while in 

 sections stained by the modified Pal method they appear as black 

 circles with a clear centre. The nerve-fibres vary in size in different 

 parts ; on the whole those which are nearest the surface of the cord are 

 larger than those nearest to the grey matter, but there is a bundle of 

 very small fibres (M, fig. 252) opposite the tip of the posterior horn. 

 The medullated fibres are supported by a peculiar reticular tissue 



