6oo 



STRUCTURE OF VERTEBRATES 





e.c. 



the branches of the orbital or external carotid as the relics of 

 those of the mandibular and premandibular segments. From 

 the spiracle back the segmental arrangement is clearer although 

 it is modified by the fact that the gills are intersegmental, and 

 in tetrapods by their loss. The simple scheme is that shown in 

 Fig. 462 A which shows an embryo shark ; in the adult, the greater 

 part of the dorsal portion of the branchial vessels disappears, 

 and is replaced b}' new efferent branchial arteries which are so 

 arranged that blood from the afferent or ventral portion of the 



vessels has to pass through the 

 capillaries of the gills before 

 reaching them. Stages in their 

 development, and in that of their 

 cross-connections, are shown in 

 Figs. 462 B-E. At the same time 

 the most dorsal parts of the 

 original vessel shift to come op- 

 posite the gill slits instead of the 

 gill bars ; they are the vessels 

 commonly dissected under the 

 name of efferent branchial arteries, 

 but also called epibranchials. In 

 teleosts a ver}^ similar arrange- 

 ment is reached by loss of the 

 original ventral portion of the 

 branchial vessels, and their re- 

 placement by new secondary 

 afferent vessels. In fishes with a 

 lung, it is supplied by a vessel from the last epibranchial. 

 In the embryos of Amphibia the branchial circulation is essen- 

 tially fish-like (Fig. 464), but in the adults, and even more in 

 reptiles, there are considerable modifications. The chief of these 

 are : a splitting of the ventral aorta into right and left halves, 

 loss of the dorsal aorta between the third and fourth arches, 

 loss of the fifth arch, loss of the dorsal part of the sixth epi- 

 branchial so that the pulmonary has no connection with the 

 dorsal aorta ; and separation of the pulmonary from the 

 ventral aorta right back to the conus. These are not all 

 present in all groups. A summary of the derivations of the 

 main vessels, and of some of the chief names used, is as 

 follows : 



Fig. 463. — A diagrammatic ventral 

 view of the dorsal arterial sj^stem 

 of a cod. 



a.mes.. Anterior mesenteric artery; a.psb., 

 afferent pseudobranchials ; c.c, carotids ; 

 c.c'., anastomosis between the internal 

 carotids which completes the circulus 

 cephalicus ; coe., coeliac ; d.ao., dorsal aorta ; 

 e.b.a., efferent branchial ; ex., orbitonasal 

 or ' external carotid ' ; e.psb., efferent 

 pseudobranchial ; i.e., internal carotid ; 

 op., ophthalmic ; p.c, orbital or ' posterior 

 carotid ; psb., pseudobranch ; sbr., supra- 

 branchial ; sd., subclavian artery. 



