ANATOMY OF THE DRILL. 443 



them and their prominent vertebral branches. In most mammals 

 the Annnlus, when present, surrounds only the subclavians. 



Intercostal ai'teries: — The upper two spaces on each side are 

 supplied by the superior intercostal branch of the subclavian 

 artery, and the internal mammary artery. The third, fourth, 

 fifth, and sixth left spaces are supplied by the intercostal arteries 

 from the aortic arch and the internal mammary artery. The 

 lower six left spaces are supplied by intercostal branches of the 

 descending aorta and the musculo-phrenic arteries. The right 

 spaces from the third to the twelfth are supplied by intercostal 

 branches of the descending aorta, internal mammary and musculo- 

 phrenic arteries. The internal mammary artery gives off a large 

 branch along the anterior rib, and a thin one along the posterior 

 rib bounding each space supplied by it. 



The aortic intercostal arteries are arranged as in those 

 Primates possessing thirteen pairs of ribs. 



The ahdominal aorta gives off branches in the following 

 order : — Inferior phrenics, cQ?liac axis, splenics, superior mesen- 

 teries, renals, spermatics, inferior mesenteric. At intervals it 

 gives off four pairs of lumbar arteries. The first left lumbar artery 

 passes postero-laterally as an ilio-lumbar artery. The aorta ends 

 by dividing into two common iliac arteries, and does not give off 

 a middle sacral (caudal) artei"y. Lateral sacral branches of the 

 hypogastrics form the caudal artery which passes through chevron 

 bones. 



2'he Veins. 



The inferior labial veins (text-fig. 18, I.P.Y) unite to form 

 a long superficial trunk which opens into the superficial temporal 

 vein (S.T.V) ; and the latter drains the frontal, temporal and 

 orbital regions, and the upper lip. The inferior labial vein com- 

 municates with the mylo-hyoid veins which drain the muscles 

 and the tongue. The mylo-hyoid trunk (M.H.V) divides into 

 two. One part enters the internal jugular vein (l.J.V), and the 

 other unites with a vessel formed by the internal maxillary 

 (I.M.V) and deep facial (D.F.V) veins. And the resulting 

 vessel opens into the internal jugular vein. The arrangements 

 and relations of these veins are shown in text-figs. 



The external jugular vein (E.J.Y) is formed on the surface of 

 the sterno-mastoid miiscle by the union of superficial temporal 

 and infeiior labial veins. It has the usual course through 

 the neck, and unites with the subclavian vein to form the in- 

 nominate vein. The internal jugular vein (I.J.Y) drains the 

 thyroid gland, and opens into the innominate vein. 



The left innominate vein cvoa&es the anterior part of the thorax 

 obliquely and unites with the more vertical right innominate 

 vein to form the superior vena cava. The latter receives the 

 vena azygos major in the usual way and ends in the antei'ior 

 part of the right auricular appendix. 



The intrathoracic part of the inferior vena cava is short. 



