274 COMPARATIVE ANATOMY 



last named, receiving an ever-diminishing quantity of blood, 

 shrivel up, the gill-slits are closed, the lungs increase in 

 size and importance, and more blood passes to them through 

 the pulmonary arteries, and also to the skin through the 



Fig. 68 



A . Diagram of the heart and chief arteries of a tadpole when three external gills 

 are present. B. Diagram showing the arrangement of the vessels when the 

 tadpole is breathing by internal gills. C. Diagram showing the derivation 

 of the adult circulation from C. ac, anterior commissural vessel ; a.cb^ 

 anterior cerebral artery; of, afferent branchial arteries; ao, dorsal aorta; 

 car, carotid artery; eg, carotid gland; cu, cutaneous artery ; rf/', ductus 

 Botalli ; ef, efferent branchial arteries; ht, heart ; hy^ efferent hyoidean 

 artery; /, connecting vessel; /, lingual artery; 7nd, efferent mandibular 

 artery ; ^c, posterior commissural vessel ; ^ic, pulmo-cutaneous arterial 

 arch; ^ui, pulmonary artery ; sys, systemic arch; ir, truncus arteriosus ; 

 V, ventricle. I-IV, branchial aortic arches. 



cutaneous branches of the pulmonary arteries. The changes 

 that lead to the adult condition are as follows : — 



The first aortic arch, as we may now call it, becomes 

 the carotid arch of the adult, the carotid gland being formed 



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