EFFECTS OF GRAVITY ON THE CIRCULATION. 681 



ally caused death. Horses have been often killed by blowing air 

 down the jugular vein. The right side of the heart is, in such cases, 

 found filled with frothy blood. The cause of death is probably due, 

 not to paralysis of the muscular fibres of the heart, but to the me- 

 chanical impossibility of the passage of frothy blood through the cap- 

 illaries of the lungs. 



The presence of valves in the veins near the heart also contributes 

 to the intermittent aid given to the venous circulation by the respira- 

 tory movements ; for, whilst they permit, during inspiration, the in- 

 flux of blood through the large veins into the chest, they prevent the 

 reflux of the blood in them during expiration, so that the balance of ad- 

 vantage is in favor of the return of the venous blood. The valves of 

 the jugular veins not only serve this purpose, but also prevent the re- 

 gurgitation of the blood towards the brain, during coughing, or other 

 efforts accompanied by violent expiration or compression of the chest. 

 This reflux motion of the blood in the great veins of the neck is shown 

 by alternating conditions of fulness and emptiness of those vessels, 

 synchronous with expiration and inspiration, producing the so-called 

 respiratory pulse. In cases in which portions of the skull have been 

 removed in the living body, and in which the veins within the cra- 

 nium, protected from atmospheric pressure at their sides, may be com- 

 pared to the tube in Sir D. Barry's experiment, an alternate rising 

 and sinking of the brain have been observed, corresponding respec- 

 tively with the movements of expiration and inspiration. These move- 

 ments must be distinguished from the slighter pulsatory movements 

 coincident with the heart's action, and dependent on the pulse of the 

 cerebral arteries. In constrictive disease of the valved orifices of the 

 heart, the return of blood into that organ from the veins, is impeded, 

 and those vessels, accordingly, become permanently distended near 

 the heart. Such disease almost always affects the orifices on the left 

 side of the heart, and its effect on the great systemic veins is commu- 

 nicated backwards, indirectly, through the pulmonary circulation. 

 Even in the healthy condition, the imperfect closure of the tricuspid 

 valve causes a venous pulse at each ventricular systole, the shock 

 being conveyed through the blood in the right auricle, and thence into 

 the veins of the neck, as far as the first set of valves. 



The effects of gravity on the venous circulation, or rather on certain 

 parts of it, have been sometimes erroneously estimated ; for it was 

 imagined that the upward current through the veins in the lower part 

 of the body, i. e., below the heart, was resisted by the weight of the 

 column of blood below that organ ; whilst the venous circulation in 

 the upper half of the body, i. e., above the heart, was thought to be 

 aided by the weight of the corresponding column of venous blood. 

 But the circulation of the blood being performed in a closed system of 

 vessels, consisting, as it were, half of arteries and half of veins, which 

 meet in the capillaries, the weight of the venous blood in the lower 

 limbs, is counterbalanced by that of the arterial blood. Hence, the 

 gravity of the venous blood does not, per se, offer such an obstacle to 

 the circulation, as requires to be overcome by the force of the heart ; 

 for the two columns of blood balance each other hsemostatically, like 



