THE BLOOD AND ITS CIRCULATION. 339 



columns. These tendinous cords are more numerous in the 

 valves of the left ventricle than in the other parts, and be- 

 ing supposed, with the valves, to resemble a mitre, are 

 named mitral valves. There are valves also in the right 

 ventricle for similar purposes, which are named tricuspid, 

 or three pointed ; also in the great artery, or aorta, and in 

 the pulmonary artery, where, having no cords, and resem- 

 bling, or supposed to do so, a half-moon, they are named 

 semilunar. The heart is enclosed in a strong membranous 

 bag, which is named pericardium, and this encloses also the 

 trunks of the veins and arteries, as well as the appendages 

 or auricles. 



The heart is a muscle, but, unlike other muscles, it is in- 

 voluntary, being altogether independent of the will, and is 

 for this purpose supplied by a peculiar set of nerves. It is 

 also furnished abundantly with blood for its support, by 

 means of arteries which are the first that are given off ; and 

 these arteries are accompanied by veins for the return of 

 the blood to its proper receptacle. 



THE CIRCULATION OF THE BLOOD 



is one of the most important processes in the animal 

 economy : when suspended for a few moments, a state of 

 insensibility is produced, and if this suspension continues a 

 little longer, death quickly supervenes. 



The heart, we have seen, consists of two halves or sides, 

 the right being devoted to the pulmonary circulation. The 

 right auricle receives from a large vein, called the vena 

 cava, the blood which has travelled throughout the system ; 

 whence it passes, by the action of the heart, into the right 

 ventricle, which by its contraction forces it into a large 

 vessel called the pulmonary artery. Thence the blood is 

 sent into the lungs and ramifies throughout its minute ves- 

 sels, where it is exposed to the action of the inspired air, 

 and becomes, by means we shall afterwards speak of, red- 

 dened and purified. This process being accomplished, the 

 blood passes into minute vessels, which, coalescing, become 

 the pulmonary veins, and through them the blood again re- 

 turns to the heart ; thus finishing the circuit of the pulmo- 

 nary circulation. 



The left auricle receives the purified blood from the pul- 

 monary veins, forces it into the left ventricle, which, con- 

 tracting, sends the vital fluid into a large strong vessel 



