528 



THE HEART OF MAMMALIA. 



r.v 



FIG. 362. HEART or A CHICK 



UPON THE SIXTH DAY OB INCUBA- 

 TION, FKOM THE VENTRAL SURFACE. 



La. left auricular appendage ; 

 r.a- right auricular appendage; 

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

 tricle ; b. truncus arteriosus. 



original 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 condition of the truucus arte- 

 riosus. 



The subsequent changes which the heart 

 undergoes are concerned more with its in- 

 ternal structure than with its external 

 shape. Indeed, during the next three clays, 

 viz. the eighth, ninth, and tenth, the ex- 

 ternal form of the heart remains nearly un- 

 altered. 



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 auricnlaris and 

 proceeding towards the opening into the 

 simis venosus. 



This latter structure gradually becomes 



aborted 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 repre- 

 sented 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 tlie 

 truncus, in the ventricular, and in the auriciilar 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). 

 and deserve to be noticed. In the primitive state the ventricular walls 

 have throughout a spongy character ; and the auriculo-ventricular valves are 

 simple membranous projections like the auriculo-ventricular valves of Fishes. 

 Soon however the spongy muscular tissue of both the ventricular and 



