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HANDBOOK OF PHYSIOLOGY. 



united by fine protoplasmic processes: by the extension of the vacuoles 

 into them, capillary tubes are gradually formed. 



Morphology. Heart. When it first appears, the heart is approxi- 

 mately tubular in form, being at first a double tube iheii & single one. It 

 receives at its two posterior angles the two omphalo-mesenteric or vitel- 

 line veins, and gives off anteriorly the primitive aorta (fig. 493). The 

 junction of the two veins which pass into the auricle becomes removed 

 farther and farther away from the heart, and the vessel thus formed is 

 called sinus venosus near to the auricle, and dnctus venosus farther 

 away, or if it be called by one name that of meatus venosus may be used. 



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

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

 same time drawn up toward 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. 



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

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



Fig. 493. Foetal heart in successive stages of development. 1, venous extremity ; 2, arterial ex- 

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



horse-shoe. The bulging out of the middle portion inferiorly gives the 

 first indication of the future form of the ventricle (fig. 493). The great 

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

 developed 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, while the ventricle becomes more and more developed in- 



