CHAPTER VI. 

 THE CIRCULATION. 



THE circulatory system is an apparatus by which the blood is trans- 

 ported to different regions of the body, and by which, after serving 

 for nutrition, absorption, or secretion, it is returned to the lungs for 

 aeration. By this movement of the blood in a continuous circuit, the 

 materials absorbed in the alimentary canal are conveyed to distant 

 parts for their nourishment and growth, the oxygen taken in by the 

 lungs is distributed throughout the body, the products of excretion 

 find their way to the outlets of the system, and the losses by exhala- 

 tion in one organ are made good by absorption in another. The me- 

 chanical function by which this is accomplished is regulated by the 

 conditions of compression, fluidity, and resistance, under which the 

 blood moves through the blood-vessels. 



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 to dif- 

 ferent parts of the body. 3d. The capillaries, a network of inosculating 

 tubules, interwoven with the substance of the tissues, bringing the 

 blood into intimate relation with their component parts ; and 4th. The 

 veins, a system of converging vessels, which 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 dependent 

 on special conditions. 



The Heart. 



The structure of the heart and its relation with the adjacent vessels, 

 is particularly connected with the activity and mechanism of respira- 

 tion. In man and mammalians, this function is very active, and is 

 performed almost exclusively by the lungs. The whole of the blood, 

 accordingly, after returning from the periphery, passes through the 

 lungs before it is again distributed to the system at large. It thus 

 traverses in succession the general circulation for the whole body, and 

 the special circulation of the lungs. The mammalian heart (Fig. 5T), 

 consists -of a right auricle and ventricle (a, 6), receiving the blood from 

 the vena cava (i), and driving it to the lungs ; and a left auricle and 

 ventricle (/, g) receiving the blood from the lungs and propelling it 

 outward through the arterial system. 



It is, therefore, a double organ, with two sets of muscular cavities, 

 right and left; its right cavities being devoted to the circulation 



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