2G4 ANATOMY OF VERTEBRATES. 



Yentricle appear, during the foetal circulation of the chick, to be 

 two : that to the right bifurcates, one division supplying the head 

 and wings, the other winds over the right bronchus : that to the 

 left also bifurcates : its left division arches over the left bronchus 

 and anastomoses vnth. the right arch a little below and behind the 

 apex of the heart : its right division arches over the back of the 

 heart, bending rather to the right, and anastomoses with the right 

 aortic arch, just above the other ' ductus arteriosus.' Each of 

 these divisions of the left primary arterial trunk sends off a branch 

 to its corresponding lung, and as the lung expands, and especially 

 begins to act as such, toward the close of incubation, the blood is 

 diverted into the pulmonary vessels, and the channels below them 

 shrink and disappear. The left primary artery is retained as the 

 trunk of the pulmonaries, and, through the changes in the interior 

 of the ventricle, this arises exclusively from the ventricle answer- 

 ing to the ' right ' in Mammals, whilst the retained aorta rises from 

 the ' left ' ventricle. It arches, however, over the right bronchus. 

 There is no left aorta in birds distinct (as in fig. 335, A, Vol. I. 

 p. 509) from the trunk (ib.^:*) which gives oiFthe artery to the left 

 lung : only one arterial trunk arises from the right ventricle 

 instead of two. 



The air-cells begin at the lower point of the lungs, like a small 

 hydatid, and extend further and further into the abdomen, before 

 the kidneys : they are at first full of a fluid; as they extend, they 

 are, as it were, squeezed among the intestines and at last fill with 

 them the whole abdomen. Soon after other air-cells are forming. 

 The lungs are, at first, free, as in Reptiles, but afterwards begin 

 to be attached to the ribs and spine. In the female embryo we 



first ' observe two oviducts, one on each 



^^^"^ side' (as in fig. 127, ^); but '^before 



hatching the right seems to decay.' ^ 



' There are two kinds of down on the 



chick, one long, which comes first, about 



two or three days before hatching; a 



second, or fine, down forms at the roots 



of the other.' ^ The little horny knob 



at the end of the beak, fig. 137, b, with 



which it breaks the shell when arrived 



Head of Gosling. Lv. ^t fuU tiuic, is also gradually forming 



into a more regular and determinate 



p;oiiit, the progress of which is seen from the first figure to the 



* AX. vol. V. p. XXV i. 



