THE CIRCULATION OF THE BLOOD 



141 



the work of the ventricles and for purposes of elementary 

 study may be mostly neglected. The student should, if pos- 

 sible, examine for himself and actually handle the auricles, 

 ventricles, and great blood vessels of a sheep's heart, which 

 in size and structure sufficiently resembles the human heart. 

 Figs. 15 and 162 should also be consulted. 



8. The mechanics of the heart beat. All force pumps con- 

 sist of two indispensable parts some device for pressing 

 upon a liquid within a chamber, and valves at the openings 

 of the chamber so arranged as to allow the passage of the 

 liquid in one direction 

 only. Each ventricle of 

 the heart is really such 

 a pump and is pro- 

 vided with two sets of 

 valves one set at the 

 inlet, between the auri- 

 cles and the ventricles, 

 and the other at the 

 arterial outlet. These 

 valves permit blood to 

 pass only from the 

 great veins through the 



auricles and 011 through the ventricles to the great arteries. 

 The contraction of the muscular wall of the ventricles pro- 

 duces pressure on the blood within their cavities ; this 

 pressure quickly and easily closes the auriculo-ventricular 

 valves and finally forces open the shut valves at the open- 

 ings of the great arteries. In this way the right ventricle 

 drives venous blood into the pulmonary artery, and the left 

 ventricle arterial blood into the aorta. With the relaxa- 

 tion of the ventricles (diastole) pressure falls within their 

 cavities, and were it not for the valves at the mouths of 

 the aorta and the pulmonary artery, blood would regurgitate, 

 or flow back, into the heart ; but this " slip " (as it is 



FIG. 68. Diagram of the action of 

 a force pump 



