PROOFS OF THE CIRCULATION. 245 



It cannot, however, flow back from the arteries on account 

 of the semilunar valves of the aorta, but it readily is pressed, 

 or in common language "sucked," thus 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 influence it, except in so far 

 as the distension or collapse of the lungs influences the 

 calibre 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 faster than it empties during expiration, thus exhib- 

 iting a sort of venous pulse. Every one, too, knows that 

 by making a violent and prolonged expiration, as exhibited 

 for example by a child with Avhooping-cough, the flow 

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

 causing them to swell up and hinder the capillary circula-, 

 tion until the person becomes " black in the face," from the 

 engorgement of the small vessels with the dark-colored ve- 

 nous 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 older 

 physiologists 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 supposed 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 



