46 SKETCH OF THE ANATOMY OF THE HORSE. 



two back-tendons, and terminates in the same manner as in the fore 

 limb. The muscle of the rear back-tendon, as we have already seen, also 

 aids in flexing the fetlock and pastern joints. 



Heart and Lungs. — The heart is a hollow muscle which acts 

 as a force pump in sending the blood through the arteries to the various 

 parts of the body. The arteries commence on the left side of the heart, 

 by one large trunk, which splits up, as it goes on, into an innumerable 

 number of small branches, that, as a rule, terminate in a microscopic 

 network of minute canals called capillaries. These canals, which 

 probably do not exceed a fortieth of an inch in length, gradually enlarge 

 on the side away from the arteries, and open out into small veins, which, 

 uniting with each other as they approach the heart, enter its right side 

 by two large branches and a few small ones. The heart now forces this 

 venous blood through the pulmonary artery to the lungs, which return 

 it by a system of capillaries and veins to the heart. We must remember 

 that the network of capillaries runs through every tissue which contains 

 blood. Thus, for instance, the blood which goes to the foot of the horse, 

 and that which proceeds to the substance of the heart itself, flow to their 

 respective parts through certain arteries, pass through a very short 

 network of capillaries, and return to the heart by veins ; a long circuit 

 being made in the first case ; a short one, in the second. 



The necessary amount of nutritive matter and water is taken up from 

 the food and drink contained in the stomach and intestines by small 

 vessels which carry it into the veins, and is thus finally brought into 

 the capillaries, the walls of which are so thin that it exudes through 

 them, and in this manner nourishes the various tissues. Before 

 food can be taken up by the system, it is necessary that it should be 

 thoroughly dissolved. As the tissues are being constantly broken up as 

 well as repaired, a system of vessels, called lymphatics, take up the waste 

 matters and carry them into the veins. When this impure or venous 

 blood is pumped by the right side of the heart into the lungs, it is acted 

 upon, there, by the oxygen of the air that is taken into the chest at each 

 breath, and is returned from the lungs to the left side of the heart in 

 a comparatively high state of purity. When the blood leaves the lungs, 

 it carries with it a certain amount of oxygen, which, uniting with the 

 broken-up material in the various tissues, converts it into products 

 which can be readily removed. 



In order that a muscle may work — i.e., contract oh being stimulated 

 by its nerves — it is necessary that it should be supphed with oxygen. 

 Also, the severer the labour, the more oxygen is used up in the muscles 

 and the larger is the supply required. As the blood has a shorter distance 

 to travel in order to make a complete circuit in the blood vessels of a 

 small animal, than in those of a large one of the same kind ; we may 



