THE BLOOD AND ITS CIRCULATION 319 



muscular apex, which points downward, 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. 



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 an upper thin-walled 

 portion with a rather large internal 



Cavity, the auricle, which Opens into Diagram showing the front half of 



a lower smaller portion with heavy the heart cut awa ^ : a > aorta; 



. !** * arteries to the lungs; la, left 



niUSCUlar walls, the ventricle. Com- auricle; lv, left ventricle ; m, tri- 



munication between auricles and cus P id valve P en ' w - bicuspid 



... . 11, , . i n or mitral valve closed ; p and r. 



ventricles is guarded by little flaps veins from the lungs ; m, right 

 or valves. The auricles receive blood auricle; n , right ventricle; 



., - , rr,, . . , v, vena cava. Arrows show di- 



from the veins. The ventricles pump rection of circulation, 

 the blood into the arteries. 



The Heart in Action. The heart is constructed on the same 

 plan as a force pump, the valves preventing the reflux of blood into 

 the auricle when it is forced out of the ventricle. Blood enters 

 the auricles from the veins because the muscles of that part of 

 the heart relax; this allows the space within the auricles to fill. 

 Almost immediately the muscles of the ventricles relax, thus allow- 

 ing blood to pass into the chambers within the ventricles. Then, 

 after a short pause, during which time the. muscles of the heart are 

 resting, a wave of muscular contraction begins in the auricles and 

 ends in the ventricles, with a sudden strong contraction which 

 forces the blood out into the arteries. Blood is kept on its course 



