SAXGUIFEROUS SYSTEM. 497 



together externally, and more compactly adjusted to each 

 other's form, than in the inferior classes. From the short- 

 ness of the trunk in this class, and the proximity of the 

 heart to its anterior part, the single great systemic artery 

 proceeding from the base of the thick conical left ventricle, 

 early divides into three principal trunks as it proceeds to the 

 right side of the body, along which the single abdominal 

 aorta descends to the pelvic region. As the arch of the 

 aorta is here directed to the right side, the first branch given 

 off is the left arteria innominata, which is the common trunk 

 of the carotid, the subclavian, the vertebral, and the thoracic 

 arteries of that side. The second branch of the aorta is the 

 arteria innominata of the right side, which divides in a 

 manner similar to that of the left. The third branch is the 

 great trunk of the descending or abdominal aorta which 

 supplies the Viscera and posterior parts, and varies less in 

 its magnitude than the two anterior aortic branches, which 

 are greatly developed in birds with large wings and powerful 

 flight, and very small in strutheous birds with heavy body 

 and feeble wings. The arteria innominata or great brachio- 

 cephalic trunk, on each side, sends forward the common 

 carotid artery which, after giving a branch to the oesophagus 

 and crop, generally gives origin to the vertebral artery. 

 The common carotids mounting along the fore part of the 

 neck, beneath the muscles, are most generally found both 

 on the left side, or with the right carotid extending along 

 the median plain, and near to the head they commonly divide 

 into the external and internal carotids, the internal being 

 here, as in the lower vertebrata, only a small branch of the 

 external trunk of the artery. Many birds have only the left 

 carotid, and a few only the right carotid artery developed 

 from the subclavian, and then dividing into two. The 

 principal branches of the external carotids are the supe- 

 rior thyroid, the lingual, the occipital which receives 

 the anastomosing end of the vertebral, the large facial which 

 forms the facial plexus behind the orbit, and the palatine 

 which often unites with the opposite, like the lingual, to 

 form a median trunk. The exterior branch of the internal 

 carotid artery forms the ophthalmic, and gives off an 

 occipital, an inferior palpebral, an ethmoidal, a lachrymal, 

 and sometimes an inferior maxillary, a harderian, a nasal, 



PART V. K K 



