464 THE ANATOMY OF THE HOKSE 



then runs along the inner side of the head of the os humeri, where it 

 receives the name of A. humeraHs, and gives off three or four muscular 

 branches, having the ulnar and spiral nerves on its inner side, and -in 

 front the radial nerve, with the humeral veins behind. Above the 

 elbow joint, and in front of the humerus, it splits into three, A. ulnaris, 

 spiralis, and radialis ; the last again dividing into two, A. plantaris 

 externa and A. P. interna, which will again be alluded to in describing 

 the anatomy of the foot. 



The common carotid artery, which is the continuation of the right A. 

 innominata, after it has given off its axillary branches, ascends along the 

 lower face of the trachea for a very short distance, and then divides into 

 the right and left carotids, which lie on each side the trachea, gradually 

 sinking deeper among the muscles of the neck till they arrive at the level 

 of the larynx, when they respectively divide into three branches — A. carotidcea 

 externa, A. occipitalis, and A. carotidcea interna. In this course they supply 

 the thyroideal artery and several small muscular branches. The external 

 carotid gives off (a) the submaxillary artery, which has a number of branches 

 supplying the muscles of the pharynx, palate, and face ; (h) the parotideal ; 

 (c) internal pterygoid ; (d) branches to the masseter and auricular muscles ; 

 and finally (e) the internal maxillary, which penetrates deeply behind the 

 lower jaw, and supplies those parts ; then going on to the eye, for which it 

 gives off a special branch, the ocular, destined to the muscles of the eye and 

 the fat in which it lies. 



The occipital artery passes backwards, deeply hidden by the muscles 

 of the neck and the transverse process of the atlas, where it unites with the 

 vertebral artery. 



The internal carotid, a comparatively small artery, ascends towards 

 the base of the skull, which it enters at the point of the petrous part of 

 the temporal bone, and supplies the brain in common with the vertebral 

 artery, with which it freely anastomoses. 



The posterior aorta must now be described. It is much longer and of 

 larger diameter than the anterior, commencing opposite the fourth dorsal 

 vertebra, where it lies at some little distance below the body of that bone. 

 Passing upwards and backwards it becomes closely connected with the 

 bodies of the vertebra, lying a little to the left, and having the oesophagus 

 and vena azygos on the right, and the thoracic duct on the left. Here it is 

 called the thoracic aorta ; but passing through the crura of the diaphragm 

 it enters the abdomen, and receives the name of abdominal aorta. The 

 thoracic division supplies small branches to the bronchi and oesophagus, as 

 well as the intercostal arteries to all but the four or five anterior intercostal 

 spaces. After passing through the diaphragm, the aorta gives off the 

 phrenic arteries right and left to the diaphragm, and then supplies the 

 important arteries of the viscera, namely : (a) the Cceliac artery, dividing 

 into the splenic, gastric, and hepatic arteries ; (h) the anterior mesenteric ; 

 (c) the renal ; (d) the spermatic ; (e) the posterior mesenteric ; (/) the lumbar 

 arteries ; and finally, just below the last lumbar vertebra, it subdivides into 

 {g) the two internal, and (h) the two external iliac arteries. In the horse there 

 is no common iliac artery, as in man, the four being given off in one gi'oup, 

 out the two internals generally forming a short continuation of the trunk. 



