CHAPTER XV. 



THE CIRCULATION. 



THE blood is a nutritious fluid, holding in solution the ingredients 

 necessary for the formation of the tissues. In all the higher animals 

 and in man, the structure of the body is compound, consisting of various 

 organs, with widely different functions, situated in different parts of the 

 frame. In the intestine the process of digestion is accomplished, and 

 the prepared ingredients of the food are thence absorbed into the blood- 

 vessels, by which they are transported to distant parts. In the lungs 

 the blood absorbs oxygen, which is afterward appropriated by the 

 tissues ; and the carbonic acid produced in the tissues is finally exhaled 

 from the lungs. In the liver, the kidneys, and the skin, other substances 

 are produced or eliminated, and these local processes are all necessary 

 to the preservation of the general organization. The circulating fluid 

 is therefore a means of transportation, by which substances produced in 

 particular organs are dispersed throughout the body, or by which sub- 

 stances produced in the tissues generally are conveyed to particular 

 organs, in order to be eliminated. 



The circulatory apparatus consists of four different parts, namely, 1st. 

 The heart ; a hollow, muscular organ, which propels the blood. 2d. The 

 arteries ; a series of branching tubes, which convey it from the heart to 

 different parts of the body. 3d. The capillaries ; a network of inoscu- 

 lating tubules, interwoven with the substance of the tissues, which bring 

 the blood into intimate contact with their component parts ; and 4th. 

 The veins ; a set of converging vessels, destined to collect the blood 

 from the capillaries, and return it to the heart. In each of these different 

 parts of the circulatory apparatus, the movement of the blood is peculiar 

 and dependent on special conditions. 



The Heart. 



The structure of the heart and of the adjacent vessels varies in dif- 

 ferent classes of animals, owing to the different arrangement of the 

 respiratory organs. 



In man and the mammalians the process of respiration is not only 

 much more active than in cold-blooded animals, but the lungs are also 

 the only special organs of aeration. The whole of the blood, accord- 

 ingly, after returning from the general system, passes through the 

 lungs before it is again distributed to the system. It thus traverses in 

 succession the general circulation for the whole body, and the special 

 circulation for the lungs. The mammalian heart (Fig. 99), consists of 

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