GENERAL PRINCIPLES OF THE CIRCULATION. 71 



pumped from the arteries past the venous valves into the veins, and the 

 arterial tension can be rapidly driven to zero. 



3. In a normal living dog, the heart is arrested by stimulation of the 

 vagus. During the standstill of the heart, the animal is alternately placed in 

 the vertical feet-down and horizontal positions. By these means the blood is 

 quickly pumped from the arteries into the veins. In the vertical feet-down 

 position, the pressure in both the aorta and the vena cava superior becomes 

 zero. On then opening the femoral artery, only a few drops of blood escape, 

 and on turning the animal into the vertical feet-up position, air is sucked into 

 the artery. 



From the consideration of these experimental facts, it is clear (1) that 

 the system is not filled to distension ; (2) that various pressures can occur in 

 different parts of it, when the heart is in arrest ; (3) that a reduction in total 

 capacity of the system need not raise the mean pressure of the whole. It is 

 owing to the great capacity of the veins that the vascular system is not filled 

 to distension, formally, the larger number of veins have a flattened instead 

 of a cylindrical shape, and, owing to this fact, a large amount of fluid can pass 

 into them, before the elasticity of their walls is brought into play. On the 

 other hand, the arteries are fashioned of a highly elastic distensile tubing, the 

 capacity of which is very small when undistended, while the elasticity is so 

 great that considerable tension is required to increase the capacity. The whole 

 of the blood within the body can pass into the roomy reservoirs of the veins, 

 and yet the walls of the veins will scarcely be thrown into tension. It is 

 partly owing to these differences that the blood vacates the arteries, and collects 

 in the veins after death. Brunner l found that, thirty seconds after complete 

 cessation of the heart-beat and respiration, the fall of pressure in the carotid 

 artery was thirty-eight to fifty-seven times as great as the fall in the jugular 

 vein. 



So long as the action of the heart and respiratory pump remain constant, 

 neither in the vena cava, nor in the pulmonary artery, is there any rise of 

 pressure during periods of vaso-constriction. 2 Thus, on excitation of the 

 peripheral end of the spinal cord (divided between the sixth and seventh 

 dorsal nerves), a rise of aortic pressure from 72 to 230 mm. Hg has been 

 recorded, while the pressure in the pulmonary artery remained at from 20 to 21 

 mm. Hg. 3 In such experiments the vena cava or pulmonary pressure does 

 not rise until the heart fails to maintain the systolic output in face of the 

 increased peripheral resistance. Similarly the fall of arterial pressure which 

 takes place on general vaso-dilatation, is not occasioned in any part by the in- 

 crease in the total capacity of the vascular system. This is further shown by the 

 fact that the vena cava pressure is generally unaltered (it may sometimes either 

 rise or fall slightly) when the spinal cord is divided in the upper dorsal region. 

 The rise of arterial pressure which takes place on constriction of vascular 

 areas, is due to the power of the heart and the increased peripheral resistance. 

 The constriction of the arterioles acts in the same way as the closing of a clip 

 placed on the aorta or on the tube (A) in Weber's model. The closing of the 

 clip scarcely diminishes the capacity of the system, but the pressure rises 

 behind the resistance. 



One of the most striking phenomena in the circulation is the comparative 

 independence of the systemic arteries, the pulmonary vessels, and the systemic 

 veins. Large oscillations of pressure in the systemic arteries produce little or 

 even no effect on the blood pressure in the venae cavae or pulmonary artery. 

 The vascular system is especially constructed as a discontinuous system, in 

 order that great changes of pressure may be brought about on the arterial side, 

 without any very great alteration of the pressures in the venous or pulmonary 



1 Ztschr.f. rat. Med., 1855, S. 336. 



2 Hill and Barnard, Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 343. 



3 Bradford and Dean, ibid., 1894, vol. xvi. p. 34, Fig. 1. 



