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HANDBOOK OF PHYSIOLOGY. 



It soon, however, becomes curved somewhat in the shape of a horse- 

 shoe, with the convexity towards the right, the venous end being at the 

 same time drawn up towards the head, so that it finally lies behind and 

 somewhat to the right of the arterial. It also becomes partly divided by 

 constrictions into three cavities. 



Of these three cavities which are developed in all Vertebrata, that at 

 the venous end is the simple auricle, with the sinus venosus, that at the 

 arterial end the bulbus arteriosus, and the middle one is the simple ven- 

 tricle. 



FIG. 474. Foetal heart in successive stages of development. 1, venous extremity; 2, arterial 

 extremity; 3, 3, pulmonary branches; 4,ductus arteriosus. (Dalton.) 



These three parts of the heart contract in succession. The auricle 

 and the bulbus arteriosus at this period lie at the extremities of the horse- 

 shoe. The bulging out of the middle portion inferiorly gives the first 

 indication of the future form of the ventricle (Fig. 475). The great 



FIG. 475. Heart of the chick at the 45th, 65th, and 85th hours of incubation. 1, the venous 

 trunks; *, the auricle; 3, the ventricle; 4, the bulbus arteriosus. (Allen Thomson.) 



curvature of the horse-shoe by the same means becomes much more de- 

 veloped than the smaller curvature between the auricle and bulbus; and 

 the two extremities, the auricle and bulb, approach each other superiorly, 

 so as to produce a greater resemblance to the later form of the heart, 

 whilst the ventricle becomes more and more developed inferiorly. The 

 heart of Fishes retains these four cavities, no further division by inter- 

 nal septa into right and left chambers taking place. In Amphibia, 

 also, the heart throughout life consists of the three muscular divisions 

 which are so early Jormed in the embryo and the sinus venosus; but the 



