216 



THE SIXTH DAY. 



[CHAP. 



FIG. 66. 



JS.CA 



DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA TOWARDS THE 

 CLOSE OF INCUBATION. 



J* 2 ) 3>4> 5- The several aortic arches. E.CA. External carotid. I.Ca. Internal 

 carotid. O. C.A. Common carotid. V.A. 'Vertebral' artery. J2.sc. Eight 

 subclavian. L.sc. Left subclavian. L.I.N. Left innominate. AO. Aorta. 

 P. A. Pulmonary arteries. R.P.A. Right arterial root, or division of 

 bulbus arteriosus, or pulmonary artery ; the left root or division, con- 

 stituting the aorta, is seen by its side. RC.b. Communication on the 

 right side between the fourth and fifth arches. L.C.b. Communicating 

 trunk between the fifth arch and the dorsal aorta. 



At the same time the connection between the third and 

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

 the passage between them becomes obliterated, so that less 

 and less of the blood which flows along the third pair of 

 arches is able to pass backwards to the hind end of the 

 body. 



On the eighth day (according to Rathke, loc. cit.) from the common root of the 

 external and internal carotids of each side, a branch (Fig. 66, V.A.) is given 

 off" which passes forwards along the neck, and ends in front by becoming 

 connected with a branch from the external carotid. This vessel is of 

 much larger calibre at its two extremities, than in the central part of its 

 course. In the adult it terminates anteriorly by anastomosing with the 

 occipital artery (a branch from the external carotid). The smaller calibre of 

 the central part is still more marked in the adult, than at the stage we are 



