CHAPTER XII. 

 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 physio- 

 logic 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 condi- 

 tions 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 varying with their 

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



The apparatus by which these results are attained is termed 

 the circulatory apparatus. This consists of a central organ, 

 the heart; a series of branching diverging tubes, the arteries; a net- 

 work of minute passageways with extremely delicate walls, the capil- 

 laries; 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 pro- 

 duced by the pumping action of the heart, though aided by other 

 forces. (See Fig. in.) 



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 mean while 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 mechanism is of primary importance. 



THE PHYSIOLOGIC ANATOMY OF THE HEART. 



The heart is a cone or pyramid shaped hollow muscular organ 

 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 fibroserous sac, the pericardium, attached to the great vessels at 

 its base. 



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