498 SAXGUIFEROUS SYSTEM. 



and a frontal branch, and terminates in the ciliary arteries 

 which ramify minutely on the choroid membrane. The 

 interior branch of the internal carotid is chiefly occupied in 

 forming the cerebral arteries, and its terminal branches 

 entering the orbit, anastomose with the divisions of the 

 ophthalmic artery. The vertebral artery ascends, with 

 the cervical portion of the sympathetic nerves, through the 

 foramina of the transverse processes of the cervical vertebrae, 

 sending off small branches to the surrounding parts in its 

 course upwards to the atlas, where it anastomoses with the 

 occipital artery, and sends in a minute twig to the medulla 

 oblongata, so that the basilar artery is here formed by 

 communicating branches of the internal carotids; the 

 blood of the vertebral arteries is diverted from the internal 

 to the external parts of the head, as in the reptiles and the 

 lower herbivorous mammalia. 



The great brachio-cephalic artery on each side, after giving 

 off the common carotid and vertebral, becomes the sub- 

 clavian and sends off an inferior thyroid to the lower larynx, 

 an internal mammary, and large thoracic and scapular arteries 

 to the surrounding muscular and cutaneous parts, especially 

 to the great pectoral muscles so important in flight, and to 

 the highly vascular subcutaneous incubating organ spread 

 over the abdomen. From the magnitude of the branches 

 thus sent from the subclavian, and from the smallness of the 

 muscular parts of the arm of birds, the axillary artery is here 

 greatly reduced, and after giving off the circumflex and deep- 

 seated branches, the brachial artery proceeds as usual along 

 the inner side of the humerus, and divides near the elbow- 

 joint into a small radial, and a larger ulnar artery which 

 furnishes considerable branches to the inter-osseous space 

 and the rest of the fore-arm, and distributes its terminal 

 twigs on the three fingers of the hand, and on the cutaneous 

 parts of the development and growth of the large feathers 

 of the wing. 



The third or right branch of the great systemic artery 

 issuing from the left ventricle, is the principal continuation 

 of the trunk, forming the descending or abdominal aorta, 

 which arches upwards and backwards on the right side 

 of the vertebral column, over the right bronchus, and 

 gradually acquires a median position under the bodies 

 of the vertebrae, along which it proceeds to the end of the 



