406 COMPARATIVE ANATOMY 



anastomoses, which later, losing the character of a network or rcte 

 mirabile, gives rise in the adult to a swelling, the carotid labyrinth 

 (so-called " carotid gland "), which consists simply of a muscular 

 vesicle with septa in its interior, and probably acts as an accessory 

 heart. Direct connections exist, however, between the second and 

 third afferent and efferent arteries, 



Towards the end of the larval period, the second efferent bran- 

 chial artery increases considerably in relative size, and the fourth 

 arterial arch also becomes larger/ By a reduction of the anasto- 

 mosis with the third arch, the fourth carries most of the blood for 

 the pulmonary artery, and the latter thus now contains more 

 venous than arterial blood. When branchial respiration ceases, the 

 anastomosis between each corresponding afferent and efferent bran- 

 chial artery no longer consists of capillaries, but a direct connection 



FIG. 309. ARTERIAL ARCHES OF AX ADULT f>rtlamainli- ,n<irn/oxn, SH<>\VN 

 SPREAD OCT. (After J. E. V. Boas.) 



"/, carotid labyrinth ; ce, external carotid ; <-i, internal carotid ; co, tr, truneus 

 arteriosus ; n-, (esophageal vessels; KA, root of the aorta; 1 4, the 

 four arterial arches. The fourth arterial arch, which gives rise to the 

 pulmonary artery (Ap), has increased considerably in size relatively, and 

 is only connected by a delicate ductus Botalli (f) with the second and third 

 arches. 



between them becomes established (Fig. 309). Finally, the con- 

 nection between the first and second arches disappears, the 

 former giving rise to the carotid and the latter forming the 

 large aortic root : an anastomosis remains throughout life, how- 

 ever, between the fourth arch, which forms the pulmonary artery, 

 and the second and third arches. This is usually spoken of as the 

 ductus liotfdli. The third arch varies greatly in its development : 

 it may be present on one side only, or may even be entirely 

 wanting (e.g. Triton). In the larv;c of Anura there are also four 

 afferent branchial arteries present on either side, but these are 

 connected with the corresponding efferent vessels by capillaries 



