370 



THE BLOOD AND ITS CIRCULATION 



Circulation of the Blood in Man. — The blood is the carrying 

 agent of the body. Like a railroad or express company, it takes 

 materials from one part of the human organism to another. This 



it does by means of the organs of 

 circulation — the heart and blood 

 vessels. These blood vessels are 

 called arteries where they carry blood 

 away from the heart, veins where they 

 bring blood back to the heart, and 

 capillaries where they connect the 

 arteries with the veinsT} The organs 

 of circulation thus forin a system of 

 connected tubes through which the 

 blood flows in a continuous stream. 



The Heart; Position, Size, Pro- 

 tection. — The heart is a cone-shaped 

 muscular organ about the size of a 

 man's fist. It is located immediately 

 above the diaphragm, and lies so that 

 the muscular apex, which points down- 

 ward, moves while beating against 

 the fifth and sixth ribs, just a little to 

 the left of the midline of the body. 

 This fact gives rise to the notion 

 that the heart is on the left side of 

 the body. The heart is surrounded by a loose membranous bag 

 called the pericardium, the inner lining of which secretes a fluid in 

 which the heart lies. When, for any reason, the pericardial fluid 

 is not secreted, inflammation arises in that region. Do you know 

 why ? ; 



Internal Structure of Heart. — If we should cut open the heart of 

 a mammal down the midline, we could divide it into a right and a 

 left side, each of which would have no internal connection with the 

 other. Each side is made up of a thin-walled portion with a rather 

 large internal cavity, the auricle, which opens into a smaller portion 

 with heavy muscular walls, the ventricle. The auricles occupy the 

 base of the cone-shaped heart ; the ventricles, the apex. Commu- 

 nication between auricles and ventricles is guarded by little flaps 



Diagram showing the front half of 

 the heart cut away : a, aorta ; 

 I, arteries to the lungs ; la, left 

 auricle ; h, left ventricle ; m, 

 tricuspid valve open ; n, bi- 

 cuspid or mitral valve closed; 

 p and ■/■, veins from the lungs; 

 ra, right auricle ; rv, right ven- 

 tricle ; », vena cava. Arrows 

 show direction of circulation. 



