238 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



auricle and prevent the regurgitation of blood when the auricle 

 contracts (Fig. 197). The right valve becomes reduced, and 

 ultimately forms the Eustachian valve, and the Thebesian which 

 is always connected with the Eustachian (Fig. 196). The part of 

 the Eustachian valve prolonged to the annulus ovalis is a new 

 formation. What becomes of the left valve is not certain ; it 

 may disappear completely, but more probably it amalgamates 

 with, and forms part of the septum primum (see Figs. 196, 

 197, and 198). The Eustachian and Thebesian valves also help 

 to indicate the part of the right auricle, which is formed 

 from the sinus venosus, for they bound the right side of the 

 entrance of the sinus to the auricle. 



The Cardiac Septa. — Every modification of the heart, from its 

 simple tubular form in fishes to its complete division into separate 

 pulmonary and systemic pumps, can be seen in the vertebrate 

 series. In amphibians the primitive auricle becomes divided 

 into right and left chambers, but the ventricle is undivided. In 

 crocodilia an incomplete ventricular septum, with a complete 

 auricular, may be seen. In birds and mammals the auricular 

 and ventricular septa are complete, and the sinus venosus becomes 

 part of the right auricle. Occasionally the septa are incompletely 

 formed in man, conditions found in the lower vertebrates being 

 thus produced. 



The division of the simple tubular heart into right and left 

 halves is rendered possible by four changes which take place in 

 its shape : 



(1) The ventricle, instead of being a bent tube, becomes 

 dilated and bag-like. 



(2) The opening of the sinus venosus in the primitive auricle 

 migrates towards its right side, and thus opens in that part of the 

 primitive auricle which becomes the right (Fig. 197). 



(3) The auricle sends out an appendix on each side; these 

 grow forwards and nearly surround the bulbus arteriosus. 



(4) The communication between the common auricle and 

 ventricle is drawn out into a tube — the auriculo-ventricular canal 

 (Fig. 198). 



The Division of the Heart is effected by five Elements or Septa 

 {Fig. 198). These are: 



1. The endo-cardial cushions. 



