CIRCULATORY ORGANS. 275 



to the radix aortae (fig. 281). These two vessels parallel each other for 

 a pa rt of their course and are connected with each other by numerous 

 capillary loops which run through the gill filaments. In passing 

 through the gills the blood loses its carbon dioxide and takes up oxygen, 

 and thus becomes converted from venous to arterial blood. In the am- 

 niotes afferent and efferent branchial arteries are never differentia ted, the 

 aortic arches being continuous from 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, 



FIG. 281. 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; hv, hepatic vein; ic, internal carotid; ec, external carotid 

 artery; i, jugular vein; /, liver; pc, postcardinal vein; sc, subclavian vein; sv, sinus venosus; 

 tr, truncus arteriosus. 



extends forward to supply the lower and a part of a upper jaw, while an 

 internal carotid artery forms an extension forward of each radix and 

 supplies the brain and face. Later their relations are such that the 

 carotids appear to arise from the first of the functional arches. 



The radices aortae of the two sides meet and fuse behind the last 

 aortic arch, forming a single tube, the dorsal aorta, which runs in the 

 middle line, dorsal to the alimentary tract, to the end of the body. The 

 fusion may also extend forward from the last aortic arch, involving the 

 whole of the radices. 



From the dorsal aorta segmental arteries extend laterally between 

 the somites, these forming the upper halves of the transverse somatic 

 vessels alluded to on page 268. To these the name of intercos- 

 tal arteries, derived from human anatomy, is given. Ventral to them 

 the aorta also gives off other arteries (nephridial arteries) to the excre- 



