352 



HUMAN PHYSIOLOGY 





membranous bag called the pericardium. The inner lining of the 

 pericardium secretes a fluid in which the heart lies. This fluid 

 prevents any friction which otherwise might arise from the con- 

 stant movement of the heart against the surrounding tissues. 

 When, for any reason, the pericardial fluid is not secreted; inflam- 

 mation arises in that region. 



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

 an ox, down the midline, we could divide it into two parts, each 



of which would have no internal connec- 

 tion with the other. Each side of the 

 heart is found to be distinct and to be 

 made up of a thin-walled portion with a 

 rather large internal cavity, the auricle, 

 and a smaller portion with heavy mus- 

 cular walls, the ventricle. The auricles 

 occupy the base of the cone-shaped heart; 

 the ventricles, the apex. The auricle of 

 the right side communicates with the ven- 

 tricle of that side. In the same manner 

 the auricle of the left side is connected 

 with the ventricle on the left side. Com- 

 munication between auricles and ventri- 

 cles is guarded by little flaps of muscle 

 called valves. The auricles receive blood 

 from the veins. The ventricles pump the 

 blood into the arteries. From each ven- 

 tricle, large arteries leave the heart; that 

 of the left side is called the aorta. 

 Through the aorta, blood passes to all 

 parts of the body. On the right side, the 

 pulmonary artery carries blood to the 

 lungs. The openings to these arteries are 

 guarded by three halfmoon-shaped flaps, which open so as to allow 

 blood to pass away from the ventricle, but not to go back into it 

 when the muscles relax. The heart is constructed on the same 

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

 auricle after it is forced out of the ventricle. 



Diagrams illustrating the force- 

 pump action of the valves 

 of the heart; A, during the 

 filling of the right ventricle; 

 B, during the contraction of 

 the ventricle. 



