70 



PHYSIOLOGY 



prevent its escape through them (Fig. 45). When the 

 chambers are distended with blood the walls contract 

 until they force the blood out, they then relax and allow 

 another supply to come in (Fig. 46). This contraction 



Fig. 46.— Diagram of heart during relaxation and contraction. A, auricle contract- 

 ing to fill ventricle ; B, auricle filling, ventricle emptying into aorta. Ao., 

 Aorta; Au.cav., Cavity of auricle; Att.v.V., Auriculo-ventricular valve; Ch.t., 

 Chordae tendinse ; V. cav., Cavity of ventricle; Pap.m., Papillary muscle. 

 (From Fitz.) 



and relaxation cause the heart beat which can be felt on 

 the left side of the chest. The thickness of the walls 

 depends on the amount of work demanded of them. 

 The auricles contract simultaneously and the blood 

 passes into the ventricles. As this requires little force, 

 the walls of the auricles are comparatively thin. The 

 ventricles contract simultaneously and the blood is sent 

 through the body. They exert greater pressure and their 

 walls are correspondingly thick. 



(2) The valves. The blood is kept flowing continu- 



