536 ANATOMY OF VERTEBRATES. 



left brachial having a distinct origin, but more remote than in the 

 Hog and Giraffe. 1 A like condition prevails in the order Carnivora, 

 ib. D. In the Otter a longer interval divides the origin of the 

 left brachial, d' , from the innominata, b ; which, after sending off 

 the left carotid, c r , is continued as a brachio-carotid trunk a short 

 way before dividing into the right carotid, c, and right brachial, d. 2 

 In the Quadrumana, from the Aye-aye up to and including 

 Hylobat.es 3 and Pithecus* the innominata, ib. r, b, gives off, first 

 the right brachial, d, and then a short bi-carotid trunk. In the 

 Hedgehogs, Moles, and Bats, there are usually two symmetrical 

 brachio-cephalics, G, b, b'. Cuvier ascribes a like condition to 

 Delphinus ; but in Phoccena the otter-type, E, is repeated, only 

 with relatively smaller brachials and larger carotids. Hyper- 

 oodon and Whales, the Seals, Beavers, Rats and most claviculate 

 Rodents, the Ornithorhynchus and Chimpanzees partake, with 

 Man, of the mode of origin shown in H : the innominata b being 

 the common trunk of the right carotid c and brachial d. The 

 same pattern obtains essentially in Sirenia, but with wider intervals 

 between b, c f ', and d r } and with a distinct origin of the left internal 

 thoracic artery, v\ 



These varieties, pretty constant in the groups they characterise, 

 are to be distinguished from the anomalies which are exceptional 

 in species. Both, and especially the latter, are explicable by 

 reference to modified or arrested stages of development ; and an 

 embryonal phase, exemplified in fig. 420, affords a ground-plan 

 on which most Mammalian arrangements of the aortic arch and 

 branches can be laid down, or from which they can be picked out. 



In the rare mammalian anomaly of a double aorta bending, one 

 over the right, the other over the left bronchus, before uniting to 

 form the descending trunk, the second of the three pairs of similar 

 vessels by which the blood passes from the heart to the dorsal 

 vessel in the embryo is retained, and such persistent aortse answer 

 to the vessels A, a', d, fig. 420 (in Saurians). When a single aorta 

 is found bending over the right bronchus, the primitive vascular 

 arch a' is retained, and A d is obliterated, as in Birds : this arrange- 

 ment is a rare anomaly, the rule in mammals being to retain the 

 left of the mid-pair of primitive vascular arches, A, D, with com- 

 plete obliteration of the right arch a'. 5 In the variety a, fig. 419, 



1 cxcv. p. 61. Cuvier seems to have found the right as well as left brachial rising 

 separately, and between them the carotids by a common trunk, xn. torn. vi. p. 112. 



2 cxcv. p. 16. s Ib. p. 15. * cxcn". p. 5. 



s I have failed to find in any embryo of bird or mammal more than three pairs of 

 primitive vascular arches, conveying the blood, in that form, from the heart to the 

 dorsal aorta. In the exceptional minority of Vertebrates, in which branchiae are deve- 



