ARTERIES OF MAMMALIA. 



537 



420 



Primitive vascular arches, as retained in 

 Saurian s. 



characteristic of the Horse and Ox, the common trunk of the fore- 

 most pair of vascular arches, fig. 420, «, is retained and lengthened, 

 the arches being modified into brachials, fig. 419, d d', and caro- 

 tids, c c f , and the communications with the succeeding arches obli- 

 terated : in most of the other varieties the communication of the 

 left of the second pair with that of the 

 first pair of primitive arches, as at fig. 

 420, d, persists and becomes the dis- 

 tinct origin of the left brachial, «*, the 

 intermediate part of the first left arch 

 being obliterated as far as the artery 

 to the head, or the trunk transmitting 

 such. But this way of explanation 

 has its limits. Most of the varieties 

 in fig. 4 1 9 bear relation to the breadth 

 of the chest, with which that of the 

 heart and aortic arch, in a measure, 

 coincides. Thus, in the non-clavi- 

 culate narrow-chested Ungulates the 

 varieties A, B, c, are met with, that 

 of A prevailing: in non-claviculate, 

 but broader-chested Unguiculates, with flexile and rotatory fore 

 limbs, the separate origin of the left brachial is more constant and 

 remote from the innominata : the same is better marked in the 

 broader-chested Swimmers (Lutra, Phocana, e), and in the cla- 

 viculate Quadrumana, F : in many Insectivora G, an analogous 

 but other arrangement prevails. In the broad-chested species 

 illustrating the variety H, the head and pectoral limbs are sup- 

 plied by three primary trunks : in the still broader and flatter- 

 chested Sirenia, I, the heart itself is able to expand laterally, 

 even to a partial severance of the ventricles, the aortic arch shows 

 its widest span, the intervals between the innominata, b, the left 

 carotid, c', and the left brachial, d' ', are longer, and the left internal 

 thoracic artery has likewise an independent origin. 



I have not met with an instance of a double aorta, or of a single 

 one arching over the right bronchus, or of the origin of the right 

 brachial from the termination of the arch, in any mammal below 

 Man : but such rare anomalies may, perhaps, be found when as 

 many individuals of the brute have been anatomised as those of 

 the human kind. 



loped from the primitive arches, four or more of these may exist. But the notion of 

 the human embryo having gills and gill-slits tickles the fancy; and so the term 

 < branchial' may long continue to be misapplied to the haemal vascular arches and 

 blastemal folds of the foetal mammal, bird and reptile. 



