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, Of 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 grv9ti 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 



