THE NERVOUS SYSTEM. 157 



Gerlach of the Sheep's brain, the breadth of the intcr>ticcs in 

 the grey 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 muter, penetrate the grey 

 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 ligamentwm denti- 

 culatum. All these vessels enter the grey 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 grey 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 homogeneous membrane ; the 

 t. media is purely muscular; and the /. 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, 

 elongated 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 measuring, in the cord, O002.2"', in 

 the brain 002'". Of the veins, the largest, for the most part, 

 do not present a trace of smooth muscle, exhibiting nothing but 

 connective tissue with nuclei, or fine elastic filaments and 

 epithelium ; in the smaller ones I have, occasionally, though 

 very rarely, observed contractile elements. 



In the ventricles of the brain there exists, under normal 

 conditions, an extremely small quantity of clear serous fluid, 

 which is manifestly secreted by the arterial plexuses, and which, 

 probably aided by the ciliary movements, assists in the nutri- 

 tion of the walls of the cavities. A second fluid, the liquor 

 cerebro-spinalis, is contained in the subarachnoid spaces above 

 described, which, according to Luschka, are lined by an 

 epithelium, and from the largest of which, extending from the 



