90 THE CIRCULATION OF THE BLOOD 



But of all the factors which influence the vaso-motor 

 centre, the one which in man is probably most often called 

 into play is the blood-supply to the brain. It is evident that 

 gravity must exert its influence upon the circulating blood. 

 According to the position of the body, the pressure in the 

 tibial artery varies from 165 mm. when the body is vertical, 

 to about 105 mm. when it is horizontal. Yet the pressure 

 in the brachial artery remains unaltered. It has already 

 been mentioned that there is no evidence of the existence 

 of vaso-motor fibres to the cerebral vessels. Alterations in 

 the intracranial blood-pressure, due to gravity, are com- 

 pensated by alteration in the facihty with which blood 

 can escape from the arteries in other parts of the body, 

 chiefly the abdomen. 



The chemical influence playing upon the centre consists 

 in the reaction of the blood. In its extreme form this is 

 seen if we asphyxiate an animal, having primarily cut the 

 vagi to eliminate the effect of any action upon the heart. 

 There occurs a rapid rise of pressure due, not to the specific 

 action of carbonic acid, but to the increase in hydrogen 

 ion concentration, for the effect can be imitated by injec- 

 tion of lactic acid into the blood-stream. 



Depressor Reflexes 



We have already seen that from the heart and begin- 

 ning of the aorta arise afferent fibres which reflexly slow 

 the heart. The same fibres reflexly produce fall of blood- 

 pressure. This is not entirely due to slowing of the heart, 

 since it occurs after section of the vagi. It is due to 

 universal vaso-dilatation. Under what circumstances is 

 this nerve normally brought into play ? Of this we have 

 no direct evidence, but it is assumed that the depressor 

 nerve-endings are sensitive to conditions of excessive tension 

 in the heart and aorta, and that when stimulated they 

 reflexly ease the strain to which these organs are put. 

 It is known that the rise in pressure during asphyxia 

 is much less when the vagi are intact than when these 



