6o4 STRUCTURE OF VERTEBRATES 



part comes from each side of the ventricle, and, by an apparently 

 pointless cross-over, that which comes from the right forms the 

 left systemic, and vice versa. Except for the connection through 

 the ductus Botalh, both carotids come entirely from the right 

 dorsal aorta ( =S3^stemic) and so connect only with the left side of 

 the heart (Fig. 465 C). In the crocodile, with its almost completely 



^ CLC 



Fig. 466. — Diagrams of the heart and chief arteries of a tadpole. — From Bourne. 



A, The vessels of a tadpole at the stage when three external gills are present ; B, the arrangement when 

 secondary gills are in use ; C, the adult arrangement. 



a.c, Anterior commissural vessel ; a.cb., anterior cerebral artery ; af., afferent branchial arteries ; ao., 

 dorsal aorta ; car., carotid artery ; e.g., carotid gland ; cu., cutaneous artery ; d.c, ductus caroticus ; 

 ef., efferent branchial arteries ; ht., heart ; hy., efferent hyoidean artery ; i., connectmg vessel ; L, lingual 

 artery; md., efferent mandibular artery; p.c, posterior commissural vessel; pl.c, pulmocutaneous 

 arch; put., pulmonary artery; sys., systemic arch; tr., truncus arteriosus; v., ventricle; I. -IV., 

 branchial aortic arches. 



divided ventricle, this means that while the carotids contain 

 oxygenated blood, the median dorsal aorta, formed by the 

 junction of the two systemics, contains mixed blood. In most 

 reptiles the left systemic is much smaller than the right. In birds 

 the ventricle is completely divided, and the left systemic is 

 suppressed, so that an exact functional similarity to mammals 

 is achieved (Fig. 325). 



VEINS 



The blood is returned to the heart by four main systems of 

 veins, those which drain the gut (hepatics and hepatic portals), 



