400 



SPECIAL HISTOLOGY. 



Fig. 152. 



arterial vessels, and ultimately subdivide, by continuous ramification, for 



the most part at acute angles into a 

 rather wide network of very fine capil- 

 laries, from which again the venous 

 radicles arise, joining so as to form the 

 well-known trunks, both on the surface 

 and in the interior (Fig. 152). The 

 gray substance is invariably much 

 more richly supplied with vessels than 

 the white, the plexus formed by them 

 being closer, and the capillary vessels 

 themselves of less calibre, to which its 

 color is in some respect due. Accord- 

 ing to E. H. Weber, the interstices of 

 the capillaries in the medullary sub- 

 stance, measure 0*0142 of a line in 

 width and 0-025 of a line in length ; 

 in an injected preparation by Gerlach 

 of the sheep's brain, the breadth of the 

 interstices in the gray substance was 

 three or four times less than in the white. 

 In the spinal cord, the disposition of the entering trunks is sometimes very 

 regularly in series. Two series of vessels of this kind exist in the bottom 

 of the anterior fissure, which, from the processes of the pia mater, pene- 

 trate the gray substance on the right and left; whilst a third series corre- 

 sponds to the posterior fissure, and others not unfrequently also to the 

 roots and the processes of the ligamentum denticulum. All these vessels 

 enter the gray substance without undergoing any direct or considerable 

 decrease in size, and there find their ultimate distribution. In the brain 

 very delicate parallel vessels are met with in the gray substance of the 

 cerebellum, less distinctly in the cerebrum and other parts. The structure 

 of the vessels is, in general, the same as elsewhere. The arteries, upon 

 their entrance into the nervous substance, possess three coats the 

 tunica adventitia, though resistant, is a thin and apparently quite homo- 

 geneous membrane ; the t. media is purely muscular ; and the t. intima 

 formed of nothing but a very delicate elastic membrane, with openings, 

 and well-marked fusiform epithelial cells. One after another of these 

 coats is gradually lost, till before the capillaries are reached, we find 

 nothing but the t. adventitia, and scattered, transversely placed, elon- 

 gated cells, with transverse nuclei and an epithelium ; with which class 

 of vessels are soon associated capillaries with a structureless membrane 

 and few or more nuclei, sometimes of great minuteness, the finest 



FIG. 152. Vessels of the cerebral substance of the Sheep, from one of Gerlach's injec- 

 tions : a, of the gray ; b, of the white substance. 



