CEREBRO-SPINAL FLUID 511 



C. THE CEREBRO- SPINAL FLUID. 



1. Distribution. — This fluid fills the pericellular spaces 

 so that the nerve cells lie bathed in it, the perivascular 

 spaces, the subarachnoid space, the ventricles of the brain 

 and the central canal of the spinal cord. At the base of the 

 brain the subarachnoid spaces filled with fluid are large and 

 they protect this important part of the nervous system 

 against injury by acting as a water cushion. 



2. Characters. — The cerebro-spinal fluid is clear and 

 transparent, with a specific gravity of 1006 to 1008. It is 

 devoid of cells and contains only traces of proteins. Its 

 Ch is practically the same as that of the blood plasma. 

 Its principal constituent is sodium chloride with sodium 

 bicarbonate and traces of phosphates, urea and dextrose. 

 It has much the composition of Locke's modification of 

 Ringer's solution, and it contains oxvgen in considerable 

 amounts. 



3. Source- — It was formerly supposed to be formed like 

 Ivmph by filtration from the blood. The amount formed 

 may be measured by inserting a cannula into the sub- 

 cerebellar space. (i) In this way it is found that, while the 

 amount produced does vary with the blood pressure, it is 

 not proportional to it. (ii) On the other hand, any increase 

 of C0.2 in the blood, the administration of such anaesthetics as 

 chloroform, and, above all, the injection of extracts of the 

 choroid plexus increase its production. 



While diffusible substances pass readily into the lymph 

 they do not all pass from the blood to the cerebro-spinal fluid. 

 Some, like urethane and alcohol, do pass, but salvarsan is held 

 back and is thus of little use in the treatment of syphilitic 

 affections of the nervous system in man. 



The fluid thus seems to be a secretion from the choroid 

 plexus. This is covered by cubical vacuolated cells, and is, 

 in fact, an inverted gland, which passes on from the blood 

 the inorganic constituents and oxygen, but which holds back 

 the proteins and many toxic substances. 



On the other hand, diffusible substances pass out readily 



