THE CIRCULATION 119 



shown in the diagram (Fig. 18), the links pass the two in 

 equal numbers, and yet there are constantly more links on 

 their way from L to R than from R to L. 



When we speak of a heart-beat we mean a co-ordinated 

 contraction of the heart's peculiar muscle. The phe- 

 nomenon includes two phases: a brief and not forcible 

 contraction of the auricles followed by a longer and much 

 more powerful closing in of the ventricles upon their cavi- 

 ties. The ventricular contraction is the essential factor in 

 propelling the blood. Valves between the auricles and the 

 ventricles permit the latter to fill during their period of 



Fig. 18. For explanation of figure see text. 



relaxation, and forbid the backward flow from the ventricles 

 when they are emptying themselves. Other valves at the 

 commencement of the aorta and the pulmonary artery 

 allow blood to pass out during each contraction of the 

 ventricles, but not to return from either artery into the 

 heart when the resting phase ensues. The action of the 

 two sets of valves is precisely on the principle of the two 

 which make a syringe-bulb an effective force-pump. The 

 right and left halves of the heart act practically at the 

 same time. 



Either ventricle when full may contain 5 or 6 fluidounces 

 of blood. As it contracts it reduces its capacity and dis- 



