CHAP, ii.] THE BRAIN. 833 



beginning of the anaemia itself. All these advantages are, 

 however, quite compatible with the coexistence of special vaso- 

 motor mechanisms. 



523. Moreover the flow of blood to, and consequent change 

 in the bulk of, the brain, and indeed the flow of blood through 

 the brain, as measured by the venous outflow, may be modified 

 independently of changes in the general blood-pressure. For 

 instance, stimulation of the motor region of the cortex quickens 

 the venous outflow, without producing any marked change in 

 the general blood-pressure ; this feature becomes very striking 

 at the onset of epileptiform convulsions when these make their 

 appearance. It is difficult not to connect such a result of func- 

 tional activity with some special vaso-motor nervous arrange- 

 ment comparable to that so obvious in the case of a secreting 

 gland. Again, it has been observed that certain drugs have an 

 effect on the volume of the brain, quite incommensurate with 

 their effect on the vaso-motor system ; thus in particular the 

 injection into the general blood stream of a weak acid produces 

 a large and immediate expansion of the brain, while the intro- 

 duction of a weak alkali similarly gives rise to similar consider- 

 able shrinking. It is suggested that these effects are produced 

 by the acid or alkali acting directly on the muscular coats of 

 the minute arteries and so leading to relaxation or contraction 

 respectively. Observations go to show that the grey matter 

 of the cortex is faintly alkaline during life and under normal 

 conditions, but becomes acid after death or when its blood- 

 supply is interfered with ; and it has been urged that nervous 

 grey matter like muscular substance developes acidity during 

 activity, as well as upon aeath, the acidity being probably due 

 in each case to some form of lactic acid. And just as it has been 

 suggested that the dilation of the minute arteries of a skeletal 

 muscle, accompanying or following the contraction of the mus- 

 cle, is brought about by the acid generated during the contrac- 

 tion causing a relaxation of the muscular coats of the minute 

 arteries, so it has been suggested that a similar acidity, the 

 product of nervous activity, similarly leads in nervous tissue to 

 a dilation of the vessels of the part. The existence of special 

 vaso-motor mechanisms would, however, afford a more satisfac- 

 tory explanation of these and other phenomena ; in spite of the 

 negative results so far obtained, the matter is obviously one 

 needing further investigation. Meanwhile we have abundant 

 evidence that, however brought about, the flow of blood through 

 the brain, and probably through particular parts of the brain, is 

 varied in accordance with the needs of the brain itself and the 

 events taking place elsewhere in the body. 



53 



