I30 THE SURGICAL ANATOMY OF THE HORSE 



only about three inches in length, and it divides into two parts, namely, 

 anterior and posterior. 



The Anterior Aorta is the smaller of the two divisions of the common 

 aorta. It curves forwards between the first pair of ribs, and like the 

 common aorta it haji a length of only about three inches, when it splits 

 up into the left axillary and innominate arteries. 



The Left Axillary Artery. — This artery is placed at a higher level than 

 the innominate, and passes forwards between the two first ribs. It is 

 related on its right side to the trachea and touches at its highest point 

 the oesophagus. It curves round the anterior border of the first rib 

 below the insertion of the scalenus muscle, and is then continued across 

 the axilla. The intra-thoracic branches of the left axillary artery are : 



(i) The Dorsal Artery^ which passes across the oesophagus and 

 longus colli muscle, to gain the second intercostal space, where it 

 splits up into branches which are distributed to the parts beneath 

 the scapula. An important branch of this artery is the subcostal, 

 which runs backwards along the under aspect of the ribs, and which 

 gives off the second, third, and fourth intercostal arteries. 



(2) The Superior Cervical Artery is given off by the axillary 

 before the latter leaves the chest. It also ascends across the 

 oesophagus and longus colli muscle to reach the first inter- 

 costal space. This vessel gives off the first intercostal artery, 

 and ultimately splits up into a great number of branches which 

 are distributed to the muscles of the upper portion of the 

 neck. It anastomoses freely with the twigs of the dorsal 

 artery. 



(3) The Vertebral Artery. — This branch is given off between the 

 first pair of ribs. It passes forwards under the transverse 

 process of the seventh cervical vertebra, and then through the 

 series of vertebral foramina to the back of the atlas, where 

 it anastomoses with the retrograde branch of the occipital 

 artery. As it passes up the neck the vertebral artery gives 



