OUTLINE OF THE STRUCTURE OF THE HORSE. 45 



of the circulation, which has been already anticipated in 

 some measure. 



After the purified blood has been returned to the left 

 auricle of the heart, by the means just indicated, it passes 

 into the left ventricle, whose thick, muscular walls contract 

 with immense power, and force it out, through the proper 

 valves, into the aorta, the great artery of the whole body. 

 This divides into two large branches, after proceeding about 

 two inches. The smaller branch is extended, by a multitude 

 of subdivisions, to every part of the head and fore extremi- 

 ties ; the larger one, in a similar manner, throughout the body 

 and hind extremities. 



The blood is now freighted with the varied elements neces- 

 sary for repairing the losses by natural decay and wear and 

 tear, which every tissue in the whole body is constantly un- 

 dergoing. This reparative process is what physiologists call 

 nutrition. It is conducted in the capillaries, the minute and 

 hair-like vessels in which the arteries every-whfere terminate. 

 Although the capillaries vary greatly in their modes of rami- 

 fication, according as they minister to gland, membrane, or 

 muscular fiber, their ofiices are the same in all locations. 

 These offices include, besides nutrition, the gathering up of 

 the worn-out, worthless particles of matter which the organs 

 of excretion are continually throwing off, through the circu- 

 lation, in all parts of the system. In the performance of these 

 duties, capillary action changes the color of the blood from 

 a scarlet to a brownish red. It also develops animal heat. 



The veins now receive this dark blood at their origin amid 

 the net- work of the capillaries, and convey it back to the 

 heart. As they approach that organ, they continue to unite, 

 and"grow larger, of , course. At length, they pour their entire 

 contents through the two vena cavce, the veins which cor- 

 respond to the great arterial branches of the. aorta, into the 

 right auricle. Only a thin wall of muscle now separates the 

 blood from its starting-point, at the outlet of the left ventricle, 

 upon the other side of the heart. But through this partition 

 there is no passage; nor is the blood ready to pass to the 



