294 



THE SIXTH DAY. 



[CHAP. 



FIG. 93. 



C.CA 



R.S.C 



DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA 



TOWARDS THE CLOSE OF INCUBATION. 



i, 2, 3, 4, 5. the several aortic arches. E.G. A. external carotid. 

 I.C. A. internal carotid. C.C.A. common carotid. V.a. ver- 

 tebral artery. R.sc. right subclavian. L.sc. left subclavian. 

 R.P., L.P. right and left pulmonary arteries. R.P.A. right 

 arterial root or division of the bulbus arteriosus, or pul- 

 monary artery ; the left root or division, constituting the 

 aorta, is seen by its side. The system of the fifth arch is 

 in lighter shading. The dotted lines shew the portions of 

 the arches which have been obliterated. 



arches in the shape of two vessels which, springing one 

 from the arch of each side, grow downwards towards the 

 lungs. At first small and narrow, these pulmonary 

 arteries, for such they are, grow rapidly larger and 

 larger, so that more and more of the blood from the 

 right ventricle is carried to the lungs. 



At the same time the connection between the third 

 and fourth pairs of arches on each side grows weaker ; so 



