THE HEART. ^\ 



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 the 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. I may mention that, 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, the capillaries also serve to 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 ^•arious tissues, converts it 

 into products which can be readily removed. I need not dwell longer on 

 this subject ; for it is fully explained in every elementary book on physiology, 

 as, for instance, that by Professor Huxley. 



In order that a muscle may work — i.e., contract on being stimulated by its 

 nerves — it is necessary that it should be supplied 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 that 

 of a large one of the same kind ; we might infer that the blood of the former 

 passes more frequently during a given space of time through the lungs, than 

 that of the latter. The truth of this supposition is fully borne out in 

 practice ; for we may observe, on an average, that the pulse of a heavy cart- 

 horse beats about thirty-five times a minute ; that of a small pony, about forty- 



