1138 THE CIRCULATION IN THE BRAIN. [BOOK HI. 



701. 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 appear- 

 ance. It is difficult not to connect such a result of functional 

 activity with some special vaso-motor nervous arrangement 

 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 introduction of a 

 weak alkali similarly gives rise to similar considerable 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. 

 In treating of the chemistry of nervous substance ( 72) we stated 

 that " the grey matter of the central nervous system is said to be 

 slightly acid during life and to become more acid after death." 

 Recent observations go to shew 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 death, 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 muscle, is brought about by 

 the acid generated during the contraction 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 satisfactory 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. 



CAMBRIDGE: PRINTED BY c. j. CLAY, M.A. & SONS, AT THE UNIVERSITY PRESS. 



