204 



Human Physiology. 



with abundance of pure air, from which it absorbs a large 

 quantity of oxygen, parting, at the same time, with a quantity 

 of carbonic acid and the various waste matters, often of a very 

 unpleasant character, which are carried out of the system with 

 the breath. 



The heart is a very simple and very perfect machine a 

 double force-pump, whose action can be understood by the 

 accompanying diagram (Fig. 45). There are, in fact, two 

 pumps, or two hearts united. The blood, returning in the 

 veins from all parts of the body, dark in colour, loaded with 

 carbonic acid and other waste, with the new matter from the 

 stomach and intestines, passes into the right auricle, by the 

 muscular contraction of which it is thrown through valves 

 which prevent its return, into the cavity below the right ven- 

 tricle. Instantly the right ventricle, or force-pump chamber of 

 this side, contracts and forces it through the pulmonary arteries 

 into every portion of the lungs, where every atom of blood is 



exposed to the action 

 of the air, and absorbs 

 oxygen and parts with 

 its waste matter through 

 membranes so thin as 

 to offer no obstacle to 

 these interchanges. In 

 Fig. 46 the blood ves- 

 sels and air vessels are 

 shown intermingled 

 but no engraving can 

 show the minute rami- 

 fications of either. The 

 air vessels are like 

 leaves on a tree, or 



grapes on their stem, with an aggregate surface of 20,000 

 square feet ; and the blood vessels are everywhere about them 



Fig. 46. HEART AND LUNGS, SHOWING 

 BLOOD AND AIR VESSELS. 



