CHAPTER XIII. 

 THE CIRCULATION OF THE BLOOD. 



Each organ and tissue of the body is the seat of a more or less active 

 metabolism, the maintenance of which is essential to its physiologic activity. 

 This metabolism is characterized by the assimilation of food materials and 

 the production of waste products; that it may be maintained it is imperative 

 that there shall be a continuous supply of the former and a continuous 

 removal of the latter. Both conditions are subserved by the blood. In 

 order, however, that this fluid may fulfil these functions it must be kept in 

 continuous movement, must flow into and out of the tissues in volumes vary- 

 ing with their activity, with a certain velocity and under a given pressure. 



The apparatus by which these results are attained is termed the circula- 

 tory apparatus. This consists of a central organ, the heart; a series of 

 branching diverging tubes, the arteries; a network of minute passageways 

 with extremely delicate walls, the capillaries; a series of converging tubes, 

 the veins. These structures are so arranged as to form a closed system of 

 vessels within which the blood is kept in continuous movement mainly by the 

 pressure produced by the pumping action of the heart, though aided by other 

 forces. (See Fig. 115.) 



In this system a particle of blood which passes any given point will 

 eventually return to the same point, no matter how intricate or tortuous the 

 route may be through which it in the meanwhile travels; for this reason the 

 blood is said to move in a circle, and the movement itself is termed the 

 circulation. 



In order to understand the reasons for the movement of the blood in one 

 direction only, as well as for many other phenomena connected with the 

 circulation, a knowledge of the structure of the heart and its internal mechan- 

 ism is of primary importance. 



THE PHYSIOLOGIC ANATOMY OF THE HEART. 



The heart is a conic or pyramid-shaped hollow muscle situated 

 in the thorax just behind the sternum. The base is directed upward and to 

 the right side; the apex downward and to the left side, extending as far as the 

 space between the cartilages of the fifth and sixth ribs. In this situation 

 the heart is enclosed and suspended in a nbro-serous sac, the pericardium, 

 attached to the great vessels at its base. 



The heart is a hollow, double muscle organ, consisting of a right and a 

 left half, separated by a musculo-membranous septum. The general 

 cavity of each side is subdivided by an incomplete transverse fibrous septum 

 into two smaller cavities, an upper and a lower, known respectively as the 

 auricle and the ventricle. The heart may therefore be said to consist of four 

 cavities, the walls of which are composed of muscle-tissue. Of these four 



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