CIRCULATORY SYSTEM 597 



vessels. From the lateral ventral aortcC, afferent brancliial arteries 

 lead to the gills, which are drained by efferent branchial arteries 

 which join to form lateral dorsal aortae. A small part of the blood 

 in these flows forwards to the head, but most goes backwards. 

 The two aortae join at some point not much farther back than 

 the heart and form a single vessel. From this arise the main 

 arteries which supply the body, paired to limbs, kidneys and 

 gonads, unpaired to the gut. 



BRANCHIAL ARTERIES 



Even in those vertebrates in which the gills persist throughout 

 life as the only respiratory organs, complications arise in the 

 branchial vessels, and as lungs progressively replace gills further 

 changes occur. Before we discuss these, some points of nomen- 

 clature must be made clear. In tetrapods the lateral ventral 

 aortae, after supplying the gills or giving off whatever branches 

 are derived from the branchial vessels, continue forward as the 

 external carotids, which supply some of the outer parts of the 

 head. In fishes these vessels, which contain only deoxygenated 

 blood, are vestigial, and the name external carotid should not 

 be used for the quite different vessels which functionally corre- 

 spond to them, but are branches of the anterior prolongations 

 of the lateral dorsal aortae. These last vessels in all vertebrates 

 supply the brain, and are called internal carotids. Each also 

 bears a branch, the orbital of fishes or stapedial of amniotes, 

 which passes near or through the hj^omandibula (^stapes), 

 and together with ophthalmic and optic branches of the internal 

 carotid supplies some of the outer parts of the head. The division 

 of the supply of these between the external carotid and the 

 dorsal arteries differs between the classes and even within a 

 class. The vessel in lizard and frog which is usually' called the 

 lingual is the external carotid (Fig. 291). In fishes the lateral 

 dorsal aort^ fuse not only posteriorly but for a short distance 

 anteriorly as well. The result is a closed loop, of varying size, 

 called the cephahc circle. It is from the anterior part of this, 

 or from a short median aorta that runs forwards out of it, that 

 the internal carotids arise (Fig. 463). - 



If it be assumed, as has been suggested on pp. 567-/-' ^^^^^ 

 the head was originally segmented nearly to its anterior end, 

 one would expect the arteries to correspond, and one may regard 



