414 COMPARATIVE ANATOMY 



318, A),; these represent the remains of the sinu-auricular valves, 

 arid in the embryo aid in conducting the blood from the right 

 atrium into the left. 1 



Great variations are seen in the mode of origin of the carotids 

 and subclavians from the arch of the aorta in Mammals. For 

 example, there may be a brachiocephalic or innominate trunk on 



FIG. 319. FIVE DIFFERENT MODES OF ORIGIN OF THE (TREAT VESSELS FROM 



THE ARCH OF THE AORTA IN MAMMALS. 



Ao, aortic arch ; <, carotids ; , subclavians ; tb, the, brachiocephalic trunk. 



either side (Fig. 319, A) ; an unpaired common brachiocephalic, 

 from which the carotid and subclavian of one or both sides arise 

 (B, c, E); or a common trunk of origin for the carotids, the 

 subclavians arising independently on either side of it (D). 



ARTERIAL SYSTEM. 



In Amphioxus, the ventral aorta, as in the Craniata, gives off 

 a series of afferent branchial arteries, which are, however, in cor- 

 respondence with the greater number of gill-clefts, much more 

 numerous than in other Vertebrates. The afferent branchial vessels 

 pass up the primary gill-bars, and give off branches to the secondary 

 gill-bars (Fig. 258). and both series of branchial vessels open 

 dorsally, as efferent branchial arteries, into a dorsal aorta on either 

 side. The latter unites with its fellow posteriorly to the pharynx 

 to form a median dorsal aorta, which gives off branches to the 

 intestine, &c. 



The essential relations of the carotid arteries, dorsal aorta, and 

 pulmonary arteries in the Craniata, as well as of the embryonic vitel- 

 line arteries, have already been dealt with. Two carotids (usually 

 described as anterior and posterior, or as internal and external) can 

 usually be recognised in Fishes, arising independently on either side 

 from the anterior afferent branchial arteries, but varying much in 

 their arrangement (Figs. 304, 320, 321 ) : there may also be a ventral 

 mandibular or lingual artery. From the Amphibia onwards, 

 internal and external carotids are formed by the bifurcation of each 

 common carotid. In these higher types, the internal carotid passes 



1 Ossifications may occur in the neighbourhood of the atrio-ventricular and 

 arterial apertures in various Ungulates. 



