588 



COMPARATIVE ANATOMY. 



There are various kinds of modifications in the Mammalia, which 



are due to similar changes in 

 the vascular trunk during em- 

 bryonic life. Among others 

 these specially affect the ter- 

 minal branches of the carotids ; 

 the internal carotid, as in 

 various Lizards and Birds, is 

 not sent to the cranial cavity 

 and sensory organs only. 



The arteries of the fore- 

 limbs arise from various and 

 very different points, so that 

 transmission appears to be of 

 less importance than adapta- 

 tion in the development of 

 these vessels. 



The trunk of the aorta is 

 continued along the vertebral 

 column, without any change 

 in character, as far as the por- 

 tion which is set apart for the 

 caudal region — caudal artery ; 

 when the tail is shortened, it 

 forms the median sacral artery. 

 When the so-called chevron 

 bones are present, the terminal 

 portion of the aorta always 

 lies in the canal formed by 

 them. But in various Fishes 

 it may be enclosed in a canal, 

 formed by processes from the 

 centra of the vertebras, in the 

 region of the trunk also ; this 

 is the case in the Sturgeon, 

 and in some Teleostei. 



The aorta gives off arteries 

 (arteriso intercostales) for the 

 metameres in regular succes- 

 sion, as well as the vessels 

 which go to the viscera, and 

 also those which go to the hind- 

 limbs, when these are well 

 developed. In Fishes there 

 is ordinarily only one large 

 arterial trunk for the viscera 

 (A. cceliaco-mesenterica) ; but 

 in some there is a mesenteric artery also. The aorta gives off a 

 larg-er number of arteries for the renal and generative organs. As 



Fig. 333. Heart and great vessels of Buteo 

 vulgaris, tr Trachea, i Crop, ae Com- 

 munication between the air-sacs and the 

 lungs. b Bursa Fabricii. ao Aortic arch. 

 aad Art. anonyma dextra. aas Art. anonyma 

 sinistra. ps Art. pulmonalis sinistra, 

 c Carotid. am Visceral artery. vci Com- 

 mencement of the inferior vena cava. 

 vcm Vena coccygeo-mescntcrica. 



