1130 THE CEREBRO-SPINAL FLUID. [BOOK HI. 



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, how- 

 ever, not the same as in life ; and the few experiments which 

 have been made seem to shew 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 solution is introduced into the subarach- 

 noid 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 affording 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 quantity 

 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 protective 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. 



