THE CAPILLAR Y BLOOD PRESSURE. 1 1 7 



inferior must be estimated. In the brain, similarly, the pressure in the 

 carotid artery and in the venous sinuses must be taken. 



If in any area the resistance in the arterioles does not vary, then 

 if both arterial and venous pressures rise together, or if one pressure 

 rise while the other remains constant, we may say with certainty that 

 the capillary pressure is increased. If both pressures fall, or if one falls 

 while the other remains constant, the capillary pressure is diminished. 

 A rise of arterial pressure, with a fall of venous pressure, or a fall of 

 arterial with a rise of venous pressure, renders the determination of the 

 capillary pressure a matter of considerable difficulty. 1 This difficulty is 

 intensified when the arterioles in the area under investigation also vary 

 in calibre. 



Variations in capillary pressure are given in the following experimental 

 conditions - (the animals are placed in the horizontal posture) : — 



Experiment I.— Section of Spinal Cord at First Dorsal Vertebra. 



Intestinal. Hepatic. Cerebral. 



Fall. Slight rise or fall. Fall. 



In this case the capacity of the venous system is greatly increased, and the 

 peripheral resistance in the arterioles greatly lowered. The pressure in the 

 intestinal and cerebral capillaries falls in consequence of the fall of arterial 

 pressure. The hepatic capillary pressure varies but slightly, for the vena cava 

 pressure remains almost unaltered. 



Experiment 2.— Arrest of the Heart by Excitation of the Vagus. 



Intestinal. Hepatic. Cerebral. 



Fall. Rise. Fall. 



The intestinal and cerebral capillary tensions fall in sequence to the enor 

 nious fall of arterial pressure. The hepatic capillary pressure rises in sequence 

 to the congestion of blood in the venous system. 



Experiment 3. — Asphyxia (Animal curarised). 



Intestinal. Hepatic. Cerebral. 



Preliminary rise, then fall. Rise. Rise. 



During asphyxia the arterial pressure rises, and finally falls ; the venous 

 pressure rises when the heart fails. The portal pressure rises at first, owing to 

 the forward expression of blood by the constricting splanchnic arterioles, then 

 falls in consequence of the diminished input. The hepatic pressure rises, in 

 sequence to the venous rise, owing to the back effect from the failure of the 

 heart to maintain the systolic output. The cerebral pressure rises greatly, 

 owing both to the arterial and venous rise of tension. 



Experiment 4. — Severe Bleeding. 



Intestinal. Hepatic. Cerebral. 



Fall. Fall. Fall. 



Here, there is a general fall of venous and arterial pressure. 



1 Bayliss and Starling, Joum. Physiol., Cambridge and London, 1894, vol. xvi. p. 160. 



2 Ibid., 1894, vol. xvi. p. 190; Hill and Bayliss, Ibid., 1895, vol. xviii. p. 334 ; Hill, 

 "Cerebral Circulation," London, 189b\ 



