526 



THE HEART OF AVES. 



nected behind with the sinus venosus. The auricular appendages grow out 

 from the auricle at a very early period. The general appearance of the heart, 

 as seen from the ventral side on the fourth day, is shewn in fig. 360. Al- 

 though the external divisions of the heart are well marked even before 

 this stage, it is not till the end of the third day that the internal partitions 

 become apparent ; and, contrary to what might have been anticipated from 

 the evolution of these parts in the lower types, the ventricular septum 

 is the first to be established. 



It commences on the third day as a crescentic ridge or fold springing 

 from the convex or ventral side of the rounded ventricular portion of the 

 heart, and on the fourth day grows rapidly across the ventricular cavity 

 towards the concave or dorsal side. It thus forms an incomplete longitu- 

 dinal partition, extending from the canalis auricularis to the commencement 

 of the truncus arteriosus, and dividing the twisted ventricular tube into 



two somewhat curved canals, one more 

 to the left and above, the other to 

 the right and below. These communi- 

 cate with each other, above the free 

 edge of the partition, along its whole 

 length. 



Externally the ventricular portion 

 as yet shews no division into two parts. 

 By the fifth day the venous end of 

 the heart, thovigh still lying somewhat 

 to the left and above, is placed as far 

 forwards as the arterial end, the whole 

 organ appearing to be drawn together. 

 The ventricular septum is complete. 



The apex of the ventricles becomes 

 more and more pointed. In the au- 

 ricular portion a small longitudinal fold 

 appears as the rudiment of the auricular septum, while in the canalis 

 auricularis, which is now at its greatest length, there is also to be seen 

 a commencement of the valvular structures tending to separate the cavity 

 of the auricles from those of the ventricles. 



About the 106th hour, a septum begins to make its appearance in the 

 truncus arteriosus in the form of a longitudinal fold, which according to 

 Tonge (No. 495) starts at the end of the truncus furthest removed from the 

 heart. It takes origin from the wall of the truncus between the fourth and 

 fifth pairs of arches, and grows downwards in such a manner as to divide 

 the truncus into two channels, one of which leads from the heart to the third 

 and fourth pairs of arches, and the other to the fifth pair. Its course down- 

 wards is not straight but spiral, and thus the two channels into which it 

 divides the truncus arteriosus wind spirally the one round the other. 



At the time when the septum is first formed, the opening of the truncus 

 arteriosus into the ventricles is narrow or slit-like, apparently in order to 

 prevent the flow of the blood back into the heart. Soon after the appear- 

 ance of the septum, however, semilunar A^alves (Tonge, No. 495) are deve- 

 loped from the wall of that portion of the truncus which lies between the 

 free edge of the septum and the cavity of the ventricles'. 



1 If Tonge is correct in his statement that the semilunar valves develop at some 



Fig. 360. Heart of a Chick on 



THE fourth day OF INCUBATION VIEWED 

 FEOM THE VENTRAL SURFACE. 



La. left auricular appendage; C.A. 

 canalis auricularis ; v. ventricle ; b. 

 truncus arteriosus. 



