5 o COMPARATIVE ANATOMY 



though open in the very young frog, is usually closed in the 

 adult and no longer admits the passage of blood. The second 

 or systemic arch runs round the oesophagus on either side 

 towards the dorsal surface, and then turns inward and back- 

 ward to join its fellow in the middle line just below the 

 vertebral column, at about the level of the sixth vertebra. The 

 right arch is directly continuous with the dorsal aorta, 

 which runs straight back beneath the vertebral column and 

 the urostyle; the left arch opens into the dorsal aorta by 

 a small opening, and is continued as the cceliaco-mesenteric 



Fig. 8. 



The Frog dissected from the right side to show the distribution of the chief 

 arteries ; semi-diagrammatic. //, heart ; tr, truncus arteriosus ; ca', carotid 

 arch ; /, lingual artery ; ca., carotid artery ; sy, systemic arch ; #?, cesophageal 

 artery ; oc.v, occipito-vertebral artery ; sc, subclavian artery, cut short ; ao, 

 dorsal aorta ; casl, coeliaco-mesenteric artery, supplying the viscera ; its 

 three main branches are indicated but all its subdivisions cannot be shown ; 

 ra, uro-genital arteries ; //, iliac arteries ; PC, pulmo-cutaneous arch, divid- 

 ing into p, the pulmonary artery supplying the lung, and cu, the cutaneous 

 artery, cut short; La, larynx; S, stomach; /, intestine; A, rectum; /?/, 

 bladder ; M, muscular abdominal wall turned back ; A", kidney. 



artery, passing to the stomach and intestines. The systemic 

 arches give off on either side before they unite to form the 

 dorsal aorta (i) a laryngeal artery, a small vessel arising 

 from the proximal part of the arch, and passing forward to 

 supply the larynx; (2) an cesophageal artery, arising from the 

 dorsal side of the upper part of the arch, and turning down- 

 ward to supply the oesophagus ; (3) an occipito-vertebral artery, 



