aMimal structure and function. 9 



the blood from the large arteries. The ftmction of the heart is to keep 

 up the circulation of the blood ; it begins to work before birth, does not 

 cease till after death, and in the meantime acts as a perfect, self-regulating 

 automatic pump. The circulation of the blood follows the course shown 

 in the accompanying diagram. 



Starting from the left auricle the blood flows down into the left 

 ventricle ; as it fills, the valves guarding the opening float up on the 

 surface of the blood, till, when they are horizontal, the meeting of their 

 edges closes it. At this moment the heart " contracts." 



This contraction is familiar to all as the " beating " of the heart ; it 

 violently shortens itself, screws itself slightly round, squeezes all its 

 chambers empty and produces a "lubb-dup" sound at each beat, then 

 relaxes, fills again, and repeats the process. The sounds maybe distinctly 

 heard and the " beat " felt by applying the ear to the chest wall. By the 

 contraction the blood in the left ventricle is forced into the main artery 

 of the body {the aorta). 



This large vessel immediately divides, one branch going to the head, 

 neck and fore limbs ; the other, the larger of the two, running along the 

 under surface of the backbone and supplying the liver, stomach, bowels, 

 and hind quarters. The finest arteries merge into the capillaries which 

 form a close-meshed network through every part of the body, and it is 

 during this capillary circulation that nourishment slips from the blood to 

 the various structures. Passing successively from the capillaries to the 

 small and the large veins, the blood reaches the right auricle, being 

 reinforced during its passage by the nourishment which has been derived 

 from the food as the result of digestion ; it then passes into the right 

 ventricle, from which it is expelled to the lungs. Here it is brought into 

 contact with the air, after admixture with which it is returned to the left 

 auricle again (see also Respiration, p. lo). 



The Lymphatic System. — This is another extensive and important The 

 system of vessels, as widely distributed as the blood-vessels, but not lymphatic 

 nearly so prominent. Its largest vessel flows into the blood stream system, 

 near the heart, and to it the whole of the body's lymphatics contribute. 

 Lymphatics are very thin-walled vessels, and as the lymph they contain is 

 colourless, they are not as a rule apparent to the eye. Their functions 

 are to collect from the bowels the goodness which is digested from the 

 food, and to prevent the accumulation of fluid in any part of the body. 

 The fluid which is thus collected is prepared and filtered by the lymph 

 glands, and ultimately poured into the large veins on the right side of the 

 heart, by the vessel alluded to above. Lymphatics only attract notice 

 as a rule when something goes wrong with them, when they are often 



