ARTERIAL PRESSURE. THE PULSE. 251 



Aspiration of the Thorax. Whenever a breath is drawn 

 the pressure of the air on the vessels inside the chest is di- 

 minished, while that on the other vessels of the Body is un- 

 affected. In consequence blood tends to flow into the chest. 

 It cannot, however, 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 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- 

 in a very noticeable way during expiration, exhibiting 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 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 ves- 

 sels 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 Bloo.cL The ancient 

 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 Phvsicians of London about 1616, demonstrated that the 

 movement of the blood was a continuous circulation as we 



