300 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



number by the more or less complete abortion of one or more pairs 

 as well as a modification of those which persist, accompanying 

 changes in the respiratory system. 



With the development of gills (ichthyopsida) each aortic arch be- 

 comes divided into two portions, an afferent branchial artery conve\- 

 ing blood from the ventral aorta to the gills and an efferent branchial 

 artery (sometimes called a branchial vein) carnjongjtjromjiie gills 

 ^to_l^^_radi2L_aortae (fig. 319, B; 320). These two vessels parallel 

 each other for a part of their course and are connected with each 

 other by numerous capillary loops which run through the gill fila- 

 ments (fig. 278). In passing through the gills the blood loses its 

 carbon dioxide and takes up oxygen, and thus becomes converted 



av 



Fig. 320. — Scheme of branchial circulation in elasmobranchs. a, atrium; aa, 

 afferent branchial arteries; av, abdominal vein; c, gill clefts; cc, common carotid; da, 

 dorsal aorta; ea, efferent branchial arteries; ec, external carotid artery; hv, hepatic 

 vein; ic, internal carotid; i, jugular vein; /, liver; pc, postcardinal vein; sc, subclavian 

 vein; sv, sinus venosus; tr, truncus arteriosus; v, ventricle; va, ventral aorta. 



from venous to arterial blood. In the amniotes afferent and efferent 

 branchial arteries are never differentiated, the aortic arches being 

 continuous from the ventral aorta to the radices aortae. 



The first of these arches (the mandibular arteries) never forms 

 afferent and efferent portions since no gills are ever developed in their 

 region. From each half of this arch an artery, the external carotid, 



^tends forwa rd t o supply the lower and a part of a upper jaw, while 

 an internal carotid artery forms an extensionforwaTd^Teach radix 



_and supplies the- hiais_§n4Jg££- (^5- S^i)- Later their relations 

 are such that the carotids appear to arise from the first of the func- 

 tional arches (fig. 319, C to H). 



As was said above, the typical number of aortic arches is six pairs, 

 this number being but rarely exceeded. In all groups except cyclos- 



