:34 



ANATOMY OF VERTEBRATES. 



418 



or muscular coat. The external coat consists of an inner stratum 

 of elastic fibres, and an outer one of the same, blended with a 

 large proportion of closely-fitted bundles of white fibres, identical 

 with those of the areolar tissue of the arterial sheath. By virtue 

 of the above-described structures arteries possess not only elasti- 

 city, but an allied power of slow and long-enduring contraction, 

 excitable by stimulus of touch, cold, and electricity during life ; 

 and lost after death. 



In the Mammalian class the aorta, fig. 418, A, bends over the 



left, not over the right bronchial 

 tube. The chief primary branches 

 of the arch are given off, not im- 

 mediately after, but at a little dis- 

 tance from, its origin ; and there is 

 less constancy in the order of their 

 origin than in birds : the phrenic 

 arteries, the coeliac axis, and the 

 superior mesenteric artery are 

 branches of the abdominal aorta, 

 which terminates, save in Mati- 

 lata, by dividing beyond the kid- 

 neys into the iliac arteries, from 

 which usually spring both the 

 femoral, «, and ischiadic branches : 

 the caudal or sacromedian artery, 

 which, in Mutilata and long-tailed 

 quadrupeds, assumes the character 

 of the continued trunk of the aorta, 

 never distributes arteries to the kid- 

 neys, rarely to the legs, as it does in 

 birds. After the arteries to the heart 

 (' coronaries ') the aortic arch sends 

 off those to the head ( f carotids') 

 and to the pectoral limbs ( f bra- 

 chials'). I use, with Barclay, the 

 latter term in preference to those by which Anthropotomy 

 designates for surgical purposes parts of the same artery, as 

 where it passes beneath a clavicle (as 'subclavian'), or sinks 

 into the arm-pit (as * axillary ') before reaching the arm. The 

 principal varieties in the origins of the large primary branches 

 of the aortic arch, characteristic of Mammalian genera or fami- 

 lies, are given in the order of their complexity in fig. 419. 



In Tapirine, Equine, Bovine, and most ruminant Ungulates, 



Central organs of circulation in Man. cclxvii. 



