8TRUCTUKE OF THE HORSE. 103 



tissues. The veins form channels back to the heart. They are 

 provided with muscular walls and valves, and are sufficiently 

 capacious to hold the total blood of the body. 



THE HEART. 



The heart is a hollow, involuntary, muscular organ, and acts 

 as a force-pump in maintaining the circulation of the blood. It 

 consists of four chambers, with contractile walls, located in the 

 chest, and surrounded by the pericardium, a fibro-serous bag, in 

 which it works. The pericardium is considerably larger than 

 the heart. It is fixed to the sternum from about the third chon- 

 dro-sternal joint to within an inch of the insertion of the dia- 

 phragm across the ensiform cartilage, while its upper and nar- 

 rower part surrounds and is attached to the great vessels con- 

 nected with the base of the heart. It consists of an external 

 fibrous layer and an internal serous sac. In form the heart re- 

 sembles a cone, slightly flattened from side to side, its base being 

 turned upwards and towards the dorsal vertebrae, from which 

 the heart is suspended by the blood vessels that spring from it; 

 the apex points downwards, backwards, and to the left side, 

 lying at about the level of the last bone of the sternum; the 

 organ extends from about the third to the sixth rib, inclusive. 



The heart contains four chambers, two auricles and two ven- 

 tricles. The cavities of the right side of the heart are, the right 

 auricle and right ventricle, the auricle being placed above the 

 ventricle. Into the right auricle open the two vena cava and the 

 coronary veins — those which supply the heart itself with blood; 

 the auriculo-ventricular opening; openings of one or two small 

 veins of right ventricle; foramina Thebesii, which are small 

 depressions, some of them transmitting minute veins. The blood 

 leaves the right auricle through the auriculo-ventricular open- 

 ing, and enters the right ventricle, which occupies the antero- 

 inferior part of the right side of the heart. Its outer walls are 

 thicker than those of the auricle. Two openings present them- 

 selves in the right ventricle, guarded by valves, the auriculo- 



