IX.] THE CAROTID ARTERIES. 295 



that less and less of the blood which flows along the 

 third pair of arches is able to pass backwards to the 

 hiad end of the body. 



The fourth arch of the right side now becomes the 

 most important of all the arches ; and nearly the whole 

 of the blood supplying the hinder parts of the body 

 passes through it. It is this arch which remaias as 

 the permanent aortic arch of the adult ; and it is im- 

 portant to notice that the arch which forms the great 

 dorsal aorta in birds is the fourth on the right side, and 

 not as ia mammals the fourth on the left side. The 

 fourth arch of the left side in birds, after giving off the 

 subclavian, is continued as an exceedingly small and 

 unimportant vessel to join the fourth right arch. It is 

 soon obliterated. 



In consequence of these changes the condition of 

 the aortic arches during the latter days of incubation, 

 before respiration by the lungs has commenced, is as 

 follows (Fig. 93). 



The first and second arches are completely ob- 

 literated. The third arch on each side is continued at 

 its dorsal end as the internal carotid, I.G.A, the con- 

 nection between it and the fourth arch having become 

 entirely obliterated. From its ventral end as the direct 

 continuation of the trunk which origiQally supplied the 

 first and second arches the external carotid, E. G.A., is 

 given off. Each pair of carotids arises therefore from a 

 common trunk — the common carotid {C.G.A.). Each 

 of these trunks gives off near its proximal end a branch, 

 the vertebral artery (V.a.). 



The common carotid on the right side comes off 

 from the fourth arch of the right side (the arch of the 



