THE VASCULAR SYSTEM 135 



cally no blood passes through them, and all the blood is com- 

 pelled to pass through the gills to reach the aorta. 



3. The Changes in the Circulation at the time of the Meta- 

 morphosis. 



At the time of the metamorphosis, however, when the 

 anterior limbs are protruded, and the tail begins to shorten, 

 these direct communications enlarge, so that an increasing 

 amount of blood takes the direct short passage, and reaches the 

 aorta without having passed through the gills. Additional 

 work is thus thrown on the lungs and skin, which consequently 

 receive a larger supply of blood : the gills rapidly atrophy, 

 though remnants of them usually persist, in a functionless con- 

 dition, until the end of the first year ; and the change from the 

 gill-breathing to the air-breathing condition is completed. 



The further changes necessary to convert the circulation into 

 that of the adult are slight. Of the four aortic arches present 

 at the metamorphosis (Fig. 34), the first, in the first branchial 

 arch, persists as the carotid arch of the adult frog ; the lingual 

 artery is a branch from the ventral end of the efierent vessel of 

 the arch, and is present from an early stage of development 

 (Fig. 33) ; and the external and internal carotid arteries are 

 already present. The carotid gland, CG, is not, as sometimes 

 stated, a persistent portion of a gill, but is formed by further 

 elaboration of the direct communication between the afferent 

 and efferent branchial vessels of the first branchial arch. 



The second aortic arch, in the second branchial arch, becomes 

 the systemic arch of the frog. Its dor.sal end remains connected 

 with the carotid arch, though the connection may in the adult 

 become closed and ligamentous. {Of. Fig. 5, p. 29.) 



The third aortic arch, in the third branchial arch, loses its 

 connection with the aorta, and finally disappears altogether.* 



The fourth aortic arch, in the fourth branchial arch, also 



* According to another view, of fairly general acceptance, the follow- 

 ing is the scheme of arterial arches in the frog : 



Arch. Embryonic. Adult. 



