826 THE CEREBRO-SPINAL FLUID. [BOOK ui. 



introduced in addition to that naturally secreted. Thus it is 

 stated that when in a dog normal saline solution is introduced 

 into the subarachnoid cavity at the lower end of the spinal cord 

 very little resorption takes place so long as the pressure remains 

 as low as about 10 c.c. of mercury; as the pressure is increased 

 beyond this resorption quickly increases. But it may be doubted 

 whether the resorption of added fluid is a fair test of the escape 

 of fluid naturally present ; and the experiment is of value rather 

 as shewing simply that there are means of escape than as afford- 

 ing a measure of the rate of escape. Besides, the immediate 

 effects of applying pressure at the caudal end of the spinal cord 

 are not the same as those of applying pressure within the skull. 

 The rate of possible escape is not without importance as 

 regards the mechanical importance of the cerebro-spinal fluid. 

 Thus it has been urged that when an extra quantity of blood is 

 driven into the skull, any injurious intercranial compression is 

 prevented, not only by the transference of a corresponding quan- 

 tity of cerebro-spinal fluid through the foramen of Majendie 

 from the cranium into the spinal canal, the walls of which are 

 less rigidly complete, but also by the direct escape of the fluid 

 from the cavity of the skull along the cranial nerves in the 

 manner described. It has also been urged that the fluid at the 

 base of the skull, in the large subarachnoid spaces of which it 

 gathers in larger quantity than elsewhere, acts as a sort of pro- 

 tective water cushion to the delicate cerebral substance, and that, 

 in general, the presence of the fluid is mechanically useful to 

 the welfare of the brain, removal of the fluid by aspiration being 

 said to lead to haemorrhage from the pia mater and to various 

 nervous disorders. But our knowledge as to the part which 

 the fluid plays is at present very imperfect ; and its very peculiar 

 chemical characters suggest that it has some chemical as well 

 at least as mechanical functions. 



