THE LYMPHATIC ARRANGEME^ 7 TS OF THE BRAIN. 731 



as the relative amount of the several constituents being about the same as 

 obtain in blood and lymph. The comparative deficiency of solids is due to 

 the scantiness of the proteids, which rarely exceed 0.1 per cent. These are 

 chiefly globulin and a form of albumose, or even peptone ; albumin is said 

 to be generally absent. The fluid, save apparently in exceptional cases, does 

 not clot, and contains neither fibrogenous factors nor fibrin ferment. It 

 very frequently contains a substance which like dextrose reduces Fehling's 

 solution but which is not sugar ; it appears to be pyrocatechin or a closely 

 allied body. 



Seeing that a fluid of such a composition is of a different nature from 

 ordinary lymph, furnished entirely in the ordinary way, we might be inclined 

 to infer that probably a very large part of the whole mass of the fluid is 

 furnished by the secreting epithelium of the choroid plexus. But it must 

 be borne in mind, that the foregoing analyses refer chiefly to fluid appearing 

 under abnormal circumstances, and it would be hazardous to draw any wide 

 inference from them. We have little or no exact experimental evidence as 

 to how much fluid is actually secreted by the choroid plexuses ; and if the 

 fluids which have been analyzed do represent a mixture of ordinary lymph 

 supplied through the pia mater with the peculiar secretion of the choroid 

 plexus and cerebro-spinal canal, some further change beyond the mere min- 

 gling of the two fluids is needed to explain the remarkable absence of albumin 

 which has been so strongly insisted upon by various authors. 



608. We may fairly suppose that during life the fluid is continually 

 being supplied, from the one source or the other ; but we have no very exact 

 knowledge as to the rate at which it is furnished. In the dog, the fluid has 

 been observed to escape at a rate varying very largely under different cir- 

 cumstances, and ranging from 1 c.c. in forty minutes to as much as 1 c.c. in 

 six minutes, the total quantity discharged in twenty-four hours varying from 

 36 c.c. to 240 c.c. In the cases of fracture of the base of the skull men- 

 tioned above, a very considerable flow has been frequently observed ; but it 

 may be doubted whether the abnormal circumstances of such cases have not 

 raised the secretion above normal. The rate of flow was found in the dog 

 to be much increased by the injection of substances (normal saline solution) 

 into the blood, but to be relatively little influenced by artificial heightening 

 of arterial pressure. This has been put forward as indicating that the fluid 

 is chiefly furnished as a secretion and not as an ordinary transudation of 

 lymph ; but it cannot be regarded as affording a valid argument. The pres- 

 sure under which the fluid exists is also very variable ; it is closely dependent 

 on the vascular arrangements of which we shall have to speak presently. 

 In the dog the average pressure has been estimated at about 10 mm. of 

 mercury. 



If the fluid is thus continually formed it mnst always find a means of 

 escape. This is probably supplied by the tubular prolongations of the sub- 

 arachnoid space along the nerve-roots ; these are continuous with the lym- 

 phatic vessels of the nerves, and so with the lymphatics of the body generally ; 

 and in the skull, the passages of this kind along the cranial nerves, especially 

 along the two optic nerves into the orbits, afford a ready means of escape. 

 It is also urged that some of the fluid escapes through the Pacchionian 

 glands directly into the blood of the venous sinuses. In a dead body fluid 

 introduced into the subarachnoid space through an opening over the bulb, 

 disappears at even a very low pressure with great rapidity. The circum- 

 stances then are, however, not the same as in life ; and the few experiments 

 which have been made seem to show that, during life, a somewhat high 

 pressure is required to secure the escape of fluid introduced in addition to 

 that naturally secreted. Thus it is stated that when in a dog normal saline 



