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CHAPTER XXVIII. 



THE CIRCULATION OF THE BLOOD. THE SANGUIVEROUS SYSTEM. 



ARTERIES. VEINS. CAPILLARIES. THE HEART, IN THE LOWER 



ANIMALS, IN MAN. PHENOMENA OF ITS ACTION. COURSE OF THE 



CIRCULATION IN MAN. FORCES BY WHICH THE CIRCULATION IS 



CARRIED ON IN THE ARTERIES, CAPILLARIES, AND VEINS. 



IT is difficult to comprehend how it escaped detection for so 

 long a time, that the complex fluid, the properties of which were 

 considered in the last chapter, is perpetually in motion. Phy- 

 siologists were not insensible of the importance of the blood to the 

 general nutrition of the body ; but of its relation to the elements 

 of the various tissues, they seem to have formed no adequate idea. 



The discovery of the circulation of the blood by our immortal 

 Harvey, and first taught by him in 1619, was, perhaps, the most 

 perfect physiological induction from well ascertained anatomical 

 facts ever made. A careful study of the anatomy of the veins and 

 of their valves, and also of the heart and its valves, and the com- 

 parison of the possible relation which these mechanical contrivances 

 in the one, might bear to those in the other, led to the inevitable 

 inference that the fluid contained in these vessels and in the heart, 

 not only moved, but also moved in a certain and uniform direction. 



The agents of the circulation of the blood, are the heart and the 

 blood-vessels : the latter being a series of tubes of various sizes and 

 structure, and of various vital endowments ; the former, a sort of 

 living forcing pump in free communication with this system of tubes, 

 which, by its unceasing action, keeps the blood in continual motion. 



We shall first examine the structure and vital endowments of 

 each of these agents of the circulation, and then inquire into the 

 part which each plays in maintaining the circulation of the blood. 



Of the Blood-vessels. The blood-vessels are of three kinds, 

 Arteries, Veins, and Capillaries. 



The Arteries are the vessels which convey the blood from the 

 heart. The etymology of the term (arjpj T^peco), shews that it 

 was adopted at a period when nothing was known as to the real 

 nature of the contents of these tubes during life. The fact that so 

 large a proportion of the arterial system is empty after death, led 



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