CHAP, x ARTERIES 451 



(Fig. 119, a. br. a, and compare Fig. no). Each afferent 

 artery passes to the corresponding gill, and there branches 

 out into smaller and smaller arteries, which finally open into 

 a network of delicate capillaries (p. 95), with which the con- 

 nective-tissue of the branchial filaments is permeated. The 

 blood in these respiratory capillaries is therefore brought 

 into close relation with the surrounding water, and as the 

 blood flows through them it exchanges its carbon dioxide 

 for oxygen, obtained from the air dissolved in the water. 



From the respiratory capillaries the blood is collected 

 into minute arteries which join into larger and larger 

 trunks, and finally unite into efferent branchial arteries (e. 

 br. a) by which the purified blood is carried from the 

 gills. The efferent arteries of the right and left sides unite 

 in a median longitudinal artery, the dorsal aorta (d. ao\ 

 which passes backwards, immediately beneath the vertebral 

 column, to the end of the tail. 



From the efferent branchial arteries and the dorsal aorta 

 are given off numerous arteries supplying the whole of the 

 body with blood. The most important of these are paired 

 Cjarotid arteries (c. a) to the head, and subclavians (scl. a) to 

 the pectoral fins ; unpaired splanchnic arteries (cl.a, ms.a\ to 

 the enteric canal, liver, pancreas, and spleen ; numerous 

 paired renals (r. a) to the kidneys, spermatic (sp. a) or 

 ovarian arteries to the gonads, and a pair of iliacs (il. a) to 

 the pelvic fins. The posterior part of the dorsal aorta, 

 supplying the tail, is contained in the haemal canal of the 

 caudal vertebrae, and is often spoken of as the caudal artery^ 

 (cd. a). 



The arrangement of the arteries in the tadpole is very similar to 

 that described above, and the diagram (Fig. 119) would serve almost 

 equally well for a tadpole as for a fish. In the former there are four 

 pairs of afferent and efferent branchial arteries (corresponding to the 



G G 2 



