528 BIOLOGICAL PHTSICS 



movement of the vaso-motor economy of the vessels, 

 so that, whenever expansion of their lumina takes place, 

 that fluid runs out of, and when contraction ensues, it 

 runs into, that spaceage, whenever and wherever deter- 

 mined by local movement, thus maintaining a condition 

 of equal pressure on the blood vasculature on the one 

 hand, and on the proper cerebral textures on the other. 

 Hence the arteries are said to enter the brain proper 

 " naked " or uncovered by their usual adventitial textures; 

 moreover, they are unaccompanied by veins, and pursue 

 their lonely course through the brain matrix, terminating 

 as usual in the capillaries, which lend themselves to the 

 transfusion of their contents into the neighbouring 

 neuroglial substance, and do not turn on themselves, but 

 continue their course until they join the venules, which 

 latter, in turn, converge to form the great venous 

 receptacles called sinuses. The sinuses are not veins, in 

 the structural sense of the term, but irregularly shaped 

 hollows, constituted by splittings and foldings of the 

 meninges at their attached margins, and in some of their 

 free expansions. They, therefore, are not contractile, but 

 are protected from mechanical pressure from the brain 

 structures to which they are related locally, by being 

 placed where these brain structures are discontinuous, as 

 they are wherever the meninges are specially attached to 

 the calvarium ; a truly remarkable provision for taking 

 advantage of the splitting up of the meningeal layers of 

 structure, and at the same time placing the sinus cavities 

 in a position as absolutely free from pressure and circu- 

 latory impediment as can be secured within the skull. 

 It may therefore be said, with all truth, that the most 

 important organ of the body is lodged in an ideally 

 protected and constructed skeletal cavity, supplied with 

 " observatory ". adjuncts of the most marvellously perfect 

 character, and has its food supply secured and supervised 

 by the most perfectly working means which it is possible 

 for the human intelligence to appreciate all which testify 

 to the great responsibility which rests on the central 

 nervous system to make the best and the utmost use 

 of the situation. It may be conceded, however, in 

 relation to a great part of the lesser, and even more, 



