176 APPLIED PHYSIOLOGY 



good the deficit. Hence it is that clinically cerebral 

 compression is marked by increased blood-pressure, and 

 this increase of pressure must be regarded as essentially 

 conservative in its action, and designed to compensate 

 for the cerebral anaemia which compression causes. It 

 is therefore not lightly to be interfered with e.g., by 

 venesection. 



The intracranial tension or pressure of the brain 

 against the skull wall is, as Hill has pointed out, purely 

 circulatory in origin, and is the same as the cerebral 

 capillary or venous pressure, and varies with every 

 change of pressure in the aorta or superior vena cava ; 

 but, like capillary pressure generally, it is more sensitive 

 to an increase of tension in the veins than in the arteries 

 (see p. 155). It is for this reason that an abrupt rise of 

 venous pressure such, for example, as occurs when any 

 effort is made with the glottis closed may easily rupture 

 a weakened cerebral capillary. This is the explanation 

 of the frequent occurrence of apoplexy during straining 

 at stool. 



