CHAP, ii.] THE BRAIN. 1129 



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 solu- 

 tion but which is not a 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 mingling of the two fluids is needed to explain the remark- 

 able absence of albumin which has been so strongly insisted upon 

 by various authors. 



696. 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 circumstances, and 

 ranging from 1 c.c. in 40 minutes to as much as 1 c.c., in 

 6 minutes, the total quantity discharged in 24 hours varying 

 from 36 c.c. to 240 c.c. In the cases of fracture of the base 

 of the skull mentioned above, a very considerable flow has been 

 frequently observed ; but it may be doubted whether the abnor- 

 mal circumstances of such cases have not raised the secretion 

 above the 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 pressure 

 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 must always find a 

 means of escape. This is probably supplied by the tubular 

 prolongations of the subarachnoid space along the nerve roots; 

 these are continuous with the lymphatic vessels of the nerves, 



