238 ZOOTOMY. 



the pectoral artery (pc.a) continues the direction of the 

 innominate and subclavian, which it nearly equals in calibre, 

 and supplies the pectoral muscles. 



240. The internal mammary artery (i.m.}, a small vessel given ofT 

 from the posterior surface of the subclavian : it passes backwards along 

 the inner surface of the ribs, and gives off transverse intercostal arteries 

 to the intercostal spaces. 



241. The arch of the aorta (a.ao\ situated immediately 

 external to the right innominate artery, of which it has the 

 appearance of being a branch : the aorta is, however, the 

 principal artery arising (see 274) from the left ventricle, 

 and the innominates are branches given off immediately 

 after its origin, and, owing to the immense size of the 

 pectoral muscles which they supply, exceeding the re- 

 mainder of the main trunk in calibre : the aorta is seen 

 to pass forwards and slightly outwards or to the right ; 

 its further course will be seen at a later stage ( 257). 



242. The pulmonary artery situated just external to 

 the left innominate, and springing from the ventral side of 

 the base of the heart (see 269) ; almost immediately after 

 its origin it divides into two trunks, the left of which (Lp.a) 

 passes forwards and outwards, parallel to the left innominate, 

 while the other (r.p.a) turns to the right and passes to the 

 dorsal side of the innominates. 



243. The right precaval veins (vena cava superior s. 

 anterior dextra,) (r.prc), a large vessel situated dorsal and 

 external to the right innominate artery : it is formed by the 

 union of three veins, the jugular (jii) ( 249) from the 

 neck, the brachial (br.v) from the wing, and the pectoral 

 (far) from the pectoral muscles : posteriorly it enters the 

 right auricle at its anterior end. 



244. The internal mammary vein (/.;), taking a parallel course to 

 the artery of the same name ( 240), and entering the pectoral vein close 

 to its junction with the jugular and subclavian. 



