642 



THE HEART OF MAMMALIA. 



r.a 



T.V 



FIG. 362. HEART OF A 

 CHICK UPON THE SIXTH DAY 

 OF INCUBATION, FROM THE 

 VENTRAL SURFACE. 



I. a. left auricular appendage ; 

 r.a. right auricular appendage ; 

 r.v. right ventricle ; l.v. left ven- 

 tricle ; b. truncus arteriosus. 



protuberance to the right now forms the apex of the ventricles, and the 

 two auricular appendages are placed at the anterior extremity of the heart. 

 The most noticeable difference (in the ventral 

 view) is the still externally undivided con- 

 dition of the truncus arteriosus. 



The subsequent changes which the heart 

 undergoes are concerned more with its in- 



o 



ternal structure than with its external shape. 



Indeed, during the next three days, viz. the 



eighth, ninth, and tenth, the external form of 



the heart remains nearly unaltered. 



In the auricular portion, however, the 



septum which commenced on the fifth day 



becomes now more conspicuous. It is placed 



vertically, and arises from the ventral wall ; 



commencing at the canalis auricularis and 



proceeding towards the opening into the 



sinus venosus. 



This latter structure gradually becomes 



reduced so as to become a special appendage 



of the right auricle. The inferior vena cava 



enters the sinus obliquely from the right, so that its blood has a tendency to 



flow towards the left auricle of the heart, which is at this time the larger of 



the two. 



The valves between the ventricles and auricles are now well developed, 



and it is about this time that the division of the truncus arteriosus into the 



aorta and pulmonary artery becomes visible from the exterior. 



By the eleventh to the thirteenth day the right auricle has become as 



large as the left, and the auricular septum much more complete, though 



there is still a small opening, the foramen ovale, by which the two cavities 

 communicate with each other. 



The most important feature in which the development of the Reptilian 

 heart differs from that of Birds is the division of the truncus into three 

 vessels, instead of two. The three vessels remain bound up in a common 

 sheath, and appear externally as a single trunk. The vessel not represented 

 in Birds is that which is continued into the left aortic arch. 



In Mammals the early stages in the development of the heart present no 

 important points of difference from those of Aves. The septa in the truncus, 

 in the ventricular, and in the auricular cavities are formed, so far as 

 is known, in the same way and at the same relative periods in both groups. 

 In the embryo Man, the Rabbit, and other Mammals the division of 

 the ventricles is made apparent externally by a deep cleft, which, though 

 evanescent in these forms, is permanent in the Dugong. 



The attachment of the auriculo-ventricular valves to the wall of the 

 ventricle, and the similar attachment of the left auriculo-ventricular valves 

 in Birds, have been especially studied by Gegenbaur and Bernays (No. 492), 



