138 



THE CIRCULATION OF THE BLOOD 



Ventricles. Each half of the heart is divided, as we 

 have seen, into two chambers, the thick walled ventricle 

 and its ear-like appendage, the auricle. Of the two ven- 

 tricles, the left pushes the blood into a large artery, 

 the aorta, which leads from it. The left ventricle has to 

 exert great pressure and its walls 

 are therefore thick. The right ven- 

 tricle, on the contrary, has the 

 lesser work of sending the blood 

 through the lungs and its walls are 

 correspondingly thinner. 



Auricles. Of the two auricles, 

 the left receives the blood from the 

 lungs ; the right, from the systemic 

 veins. They then push it for- 

 ward into their respective ventri- 

 cles when these are relaxed to re- 

 ceive it. The auricles serve mainly 

 as reservoirs for holding the blood 

 while the ventricles are contracting. 

 As they have to do but little work, their muscular walls 

 are comparatively thin. 



Auriculo-ventricular valves. Lying between each 

 auricle and its corresponding ventricle, there is a strong 

 thin flexible partition which is attached by its outer 

 edge to the wall of the ventricle. It is divided into 

 three flaps on the right side of the heart, into two 

 on the left. Slender tendons arise from the free edges 

 of the flaps and are attached by means of muscular pro- 

 jections, the papillary muscles, to the inner walls of the 

 ventricles. The purpose of these tendons is to hold the 

 flaps in place, so that they will act as valves to prevent 

 the blood from going back into the auricle when the 

 ventricles contract. When the ventricles relax, they 



FIG. 80 Transverse sec- 

 tion through the middle 

 of the ventricles of a 

 dog's heart when full and 

 when contracted. 



