18 METABOLISM IN THE NORMAL FUNCTIONAL STATE 



equally well be interpreted in terms of entry from the blood via the 

 capillaries. Data presented by Bakay (1954) would seem to suggest 

 this, for after an intravenous injection of radiophosphate the 

 specific radioactivities of the cerebral cortex and of the ventricular 

 walls were approximately equal over a period of some 21 days while 

 the specific radioactivity of the cerebrospinal fluid was always lower 

 than that of either surface. Cerebrospinal fluid was sampled from 

 the cisterna magna and owing to the slow passage from the ven- 

 tricles it is possible that in the early periods following the injection 

 of radioactive phosphate, the fluid in the ventricles was of a higher 

 specific radioactivity than that in the cistern. However, if the 

 capillary route is to be excluded, this does not explain why the 

 activities found in the cerebral cortex were higher than in the 

 cisternal fluid. The present evidence would appear to be more in 

 favour of entry via the capillaries than the cerebrospinal fluid, but 

 the critical experiments have still to be done. 



Penetration of phosphate from the surfaces to the depths of the 

 brain is slow. Following intravenous injection in the cat, pene- 

 tration from the linings of the ventricles and from the cortex to the 

 centre of the brain took 9-21 days for completion as judged by 

 radioautographs (Bakay, 1954). In contrast intracisternal injection 

 led to an apparently much more rapid penetration, being complete 

 within 1 hr, judged by the same technique (Bakay, 1951) though 

 here also the area of brain closest to the site of injection incorporated 

 radioactive phosphate faster than the other areas. This uneven 

 rate of penetration would appear to set a limit to the use that can 

 be made of intracisternal injection as a means of studying changes 

 in the distribution of radioactive phosphorus between different 

 phosphates samples from the whole brain. Bakay's experiments 

 have been criticized by Selverstone (1958) on the grounds that the 

 quantities of radioactive phosphate injected into the cistern were 

 grossly disproportionate to the amount appearing in the brain 

 following an intravenous injection. Selverstone found that the 

 ratio of the doses chosen by Bakay produced a concentration of 

 radioactive phosphate in the cisterna magna some fifteen times 

 higher when given by intracisternal injection than when given 

 intravenously. When comparable concentrations were presented 

 by each route it was found that the overall exchange of phosphate 

 between the blood and the brain was not much slower than 

 exchange between the cerebrospinal fluid and the brain. Intra- 



