OF BLOOD-PRESSURE 65 



capillaries of the brain. Both of these actually 

 occur. 



When it is desired to warm or light one particular 

 room in a house where the supply of gas is insuffi- 

 cient, all unnecessary lights elsewhere are turned 

 down. It is not possible to dilate in this way the 

 pipes passing to the room in question, but the rate 

 of flow of gas is increased. Similarly, when more 

 blood is required by the brain, the vasomotor centre 

 issues a universal vasoconstrictor impulse. This is 

 not able to constrict the arteries in the brain, because 

 the nerve fibres which undoubtedly run with them 

 appear to have little or no vasomotor control over 

 them. Hence these arteries are distended by the 

 pressure of blood, the cranial veins are expressed, and 

 the rate of flow increases. 



Mental activity, therefore, should be accompanied 

 by a rise of blood-pressure, and sleep by a fall. 

 Further, during sleep the volume of a limb, as 

 measured by a plethysmograph, should increase. 

 Both these statements are found to be true in fact. 

 Indeed, if a man go to sleep accurately balanced on 

 a delicate see-saw, during deep sleep the legs are 

 lowered ; when he wakes, the equilibrium is regained. 



When part of the available cranial space is occupied 

 by blood-clot, depressed bone, or inflammatory 

 products, it is at the expense of the volume of 

 blood. An adequate circulation through the medulla 

 can be maintained in that case only by an exaggera- 

 tion of the process just described, and the blood- 

 pressure will necessarily be high. Any attempt to 

 bring it down by violent purgation, drugs, or, worse 



5 



