50 STRUCTURE AND ECONOMY OF THE HORSE. 



and resembling in shape a half moon, they are named serai- 

 lunar. 



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 invo- 

 luntary, 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 arc the first that are given oft', and these arteries 

 are accompanied by veins, for the return of the blood to its 

 proper receptacle. — Ed.] 



CHAP. XL 



ON TUE CIRCULATION OF THE BLOOD, 



[The right side of the heart being devoted to the pulmonary 

 circulation, its auricle receives the blood which has travelled 

 throughout the system from a large vein, called the vena cava, 

 Avhence it passes, by the action of the heart, into the right ven- 

 tricle, which, by its contraction, forces it into a large vessel, 

 called the pulmonary artery. 



Thence the blood is sent into the lungs, and ramifies through- 

 out its minute vessels, where it is exposed to the action of the 

 inspired air, and becomes, by means we shall afterwards speak 

 of, reddened and purified. This process being accomplished, the 

 blood passes into minute veins, which, coalescing, become the 

 pulmonary veins (in the horse eight in number), and through 

 them the blood again returns to the heart. This is the pul- 

 monary circulation. 



The left auricle, receiving the purified blood from the pul- 

 monary veins, forces it into the left ventricle, which, contracting, 

 sends the vital fluid into a large strong vessel, called the aorta, 

 whence it passes into smaller arteries, to be distributed through- 

 out the whole system. The remote divisions of the arteries are 

 called the capillary vessels, and in them the blood, after having 

 accomplished its purposes and conveyed nourishment to all 

 parts, becomes black and impure, and in this state enters the 

 capillary veins ; which, conjoining and increasing in size, and 

 diminishing in number, convey the blood again to the right 

 auricle of the heart. Just before it enters the heart, it receives 

 a supply of a milky fluid, called chyle, which is extracted from 

 the food, absorbed by certain small vessels, called lacteals, and 

 conveyed by a specific channel into the vena cava. Such, then, 

 is the circle, or rather the double circle, which the blood takes, 



