THE HEART AND ITS VESSELS. 



283 



from the third branchial vein (Fig. 227, 4, Ap}. The pulmonary 

 artery therefore contains far more arterial than venous blood, and 

 thus the lungs of the Salamander larva, like the air-bladder of 

 Fishes, can have no important respiratory function. 



The internal carotid (ci) arises from the first branchial vein, 

 towards the middle line, the external carotid (c'c) coming off 

 further outwards. 



The latter, as it passes forwards, becomes connected with the first 

 branchial arch (1) by net-like anastomoses (t), and these give rise 

 later to the so-called carotid gland 1 of the adult, which functions 

 as an accessory heart. Direct connections exist between the second 

 and third branchial arteries and the corresponding veins (see 

 227, a, a). 



t 



. 



KJC. 227. THE ARTERIAL AKCHE.S OF THE LARVA OF A SALAMANDER. (Sliglitlj 



diagrammatic. 



(After J. E. V. Boas.) 



tr, conns arteriosns ; 1 to 3, the three liranchial arteries ; / to III, the corresponding 

 branchial veins ; 4> the fourth arterial arch, which becomes connected with the 

 pulmonary artery (Ap) ; a, a, direct anastomoses between the second and third 

 branchial arteries and branchial veins; ce, external carotid ; ci, internal carotid : 

 t, net-like anastomoses between the external carotid and the first branchial 

 artery, which give rise later to the carotid gland ; UA, aortic roots ; Ao, dorsal 

 aorta. The arrows show the course which the blood takes. 



Towards the end of the larval period, the second branchial vein 

 increases considerably in relative size, and the fourth arterial arch 

 also becomes larger. By a reduction of the anastomosis with the 

 third branchial vein, the fourth arterial arch furnishes the main 

 amount of blood for the pulmonary artery, and the latter thus 

 contains far more arterial than venous blood. When branchial 

 respiration ceases, the anastomoses between the branchial arteries 

 and veins no longer consist of capillaries, but a direct connection 

 between them becomes established (Fig. 228, 2, 3, 4}- Finally, the 

 connection between the first and second branchial arches disappears, 



1 The " carotid gland " loses its character as a rete mirabile (comp. p. 292), 

 and in the adult consists simply of a muscular vesicle with septa in its interior. 



