OUTLINE OF THE STRUCTURE OF THE HORSE. 47 



which convey it. There is no tissue an3nvhere in the entire 

 system which does not receive from the blood the elements 

 essential for its development and health — ^the materials, in 

 fact, of which it is composed. After its work is done here, 

 the blood becomes the scavenger of the body, collecting the 

 impure and deleterious excretions, and returning with them 

 to the heart and thence to the lungs, there to be exhaled in 

 the breath, as already described. 



The importance of the arterial flow can hardly be estimated. 

 Without it no function could be discharged, and, in fact, life 

 itself would be extinct. Yet it may be made the sure means 

 of disease, by forcing it into harmful and poisonous sub- 

 stances. Great and constant attention should be exercised 

 in regard to the materials introduced into the stomach, either 

 as food or medicine, since they so surely and speedily fin^ 

 their way into the stomach. 



THE HEART. 



Of the heart, much has necessarily been said in the pre- 

 ceding section. All that now remains to be added is a de- 

 scription of its difterent parts, and their oflices. It occupies 

 the space between the lungs denominated the mediastinum, 

 and is invested by a double membrane of its own, called the 

 pericardium. This forms a little sac, whose of&ce it is to sup- 

 port the heart in its natural position, and prevent friction 

 between the heart and the surrounding parts. 



Four cavities occupy the spaces within the walls of the 

 heart — two above, called auricles, from their fancied resem- 

 blance in form to the ear, and two below, called ventricles. 

 The walls of each ventricle are much thicker than those of 

 the auricle upon the same side, and also in the left ventricle 

 than in the right. Why this difference is quite plain, from 

 the uses of each of these parts, as explained in the last sec- 

 tion. The ventricles are in that part of the heart toward its 

 apex, or point. 



Into the right auricle open the two vena cavse and the cor- 

 onary veins, — ^those which supply the heart itself with blood. 





