26 Hugh Davson 



type of fluid, the aqueous humour — similar because both are 

 speciahzed tissue fluids fifling cavities and being virtually 

 free from protein. By summing the cations and anions it 

 becomes clear that the cerebrospinal fluid has a higher con- 

 centration than the plasma or the aqueous humour ; allowance 

 must be made for the lower concentrations of glucose and urea 

 in the cerebrospinal fluid, a difference amounting to some 

 5 m-mole; thus the cerebrospinal fluid is hyperosmotic by 

 some 9 m-mole. The amount is small — some 3 per cent — never- 

 theless it represents a diff*erence of osmotic pressure of some 

 160 mm. Hg, and it is presumably because the fluid is able to 

 drain away easily from its cavities that this pressure does not 

 develop, i.e. the difference in osmolarity is reflected in a 

 continuous influx of water from the blood rather than in the 

 development of a pressure, such as would happen were the 

 system completely closed. However, the really significant 

 point to be made in this connexion is that the cerebrospinal 

 fluid lies in such close relationship with the brain and cord 

 that it seems most unlikely, having regard to the rapidity 

 with which water may exchange between the two, that a 

 diff'erence of osmolarity could be maintained. That is, if the 

 cerebrospinal fluid is, indeed, hypertonic to plasma, then so 

 must the tissue of the brain and cord be. If this is true, 

 then we may postulate one of two things: either a water 

 pump that drives water out of the nerve cells into the 

 extracellular fluid where it passes back into the blood; 

 or alternatively the elaboration, by the capillaries of the 

 nervous tissue, of a hyperosmotic extracellular fluid. The 

 capillaries in this region of the body are certainly different 

 from those in the rest of the body and are responsible, pre- 

 sumably, for the so-called "blood-brain barrier"; to attribute 

 secretory activity to their endothelium is by no means an 

 unreasonable proposition. The important point to be made 

 here is that the diff'erence of osmolarity is small and thus 

 requires highly accurate analysis for its demonstration. Why 

 the cerebrospinal fluid and nervous tissue should have this 

 higher osmolarity is not clear; according to Flexner (1938), 



