334 THE HUMAN BODY 



consequence blood tends to flow into the chest. It cannot, how- 

 ever, flow back from the arteries on account of the semilunar 

 valves of the aorta, but it can readily be pressed, or in common 

 language "sucked," into the great veins close to the heart and 

 into the right auricle of the latter. The details of this action must 

 be omitted until the respiratory mechanism has been considered. 

 All parts of the pulmonary circuit being within the thorax, the 

 respiratory movements do not directly influence it, except in so 

 far as the distention or collapse of the lungs alters the caliber of 

 their vessels. 



The considerable influence of the respiratory movements upon 

 the venous circulation can be readily observed. In thin persons 

 the jugular vein in the neck can often be seen to empty rapidly 

 and collapse during inspiration, and fill up in a very noticeable 

 way during expiration, exhibiting a sort of venous pulse. Every 

 one, too, knows that by making a violent and prolonged expira- 

 tion, as exhibited for example by a child with whooping-cough, 

 the flow in all the veins of the head and neck may be checked, 

 causing them to swell up and hinder the capillary circulation until 

 the person becomes " black in the face," from the engorgement of 

 the small vessels with dark-colored venous blood. 



In diseases of the tricuspid valve another form of venous pulse 

 is often seen in the superficial veins of the neck, since at each 

 contraction of the right ventricle some blood is driven back 

 through the right auricle into the veins. 



Proofs of the Circulation of the Blood. The ancient physiolo- 

 gists believed that the movement of the blood was an ebb and 

 flow, to and from each side of the heart, and out and in by both 

 arteries and veins. They had no idea of a circulation, but thought 

 pure blood was formed in the lungs and impure in the liver, and 

 that these partially mixed in the heart through minute pores sup- 

 posed to exist in the septum. Servetus, who was burnt alive by 

 Calvin in 1553, first stated that there was a continuous passage 

 through the lungs from the pulmonary artery to the pulmonary 

 veins, but the great Englishman Harvey first, in lectures delivered 

 in the College of Physicians of London about 1616, demonstrated 

 that the movement of the blood was a continuous circulation as 

 we now know it, and so laid the foundation of modern Physi- 

 ology. In his time,- however, the capillary vessels had not been 



