Chap, ii.] THE BRAIX. 825 



mind, that the foregoing analyses refer chiefly to fluid appear- 

 ing 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 sup- 

 plied 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 remarkable absence of albumin which has been so strongly 

 insisted upon by various authors. 



§ 518. 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 pro- 

 longations of the subarachnoid space along the nerve roots; 

 these are continuous with the lymphatic 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 circumstances then 

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

 which have been made seem to shew that, during life, a some- 

 what high pressure is required to secure the escape of fluid 



