896 HUMAN ANATOMY. 



vene between it and the hemiazygos, and the veins of these then open directly into the azygos, 

 passing, each independently, across the vertebral column beneath the aorta and oesophagus 

 Absence of the accessory hemiazygos has been observed, the upper six or eight inter- 

 costal veins uniting to form a common ascending trunk which opens into the left innominate. 

 In all probability, however, this common ascending stem is properly to be regarded as the 

 accessory hemiazygos, whose normal connection with the innominate has increased in size while 

 Its connection v\ith the azygos or hemiazygos has either degenerated or failed to form. 



The Intercostal Veins. 



The intercostal veins (w. intercostales) (Fig. 765), sometimes designated as 

 posterior intercostal as distinguished from the anterior intercostal tributaries of the 

 internal mammary vein, accompany the intercostal arteries and are twelve in number 

 on each side, one occurring in each intercostal space and one, sometimes termed the 

 subcostal vein, running along the lower border of each twelfth rib. They lie along 

 the upper border of the spaces to which they belong, in a groove on the lower border 

 of the rib, and are above the corresponding arteries. The upper nine or ten veins 

 open anteriorly into the internal mammary or musculo-phrenic veins, but the lower 

 three or two, which are somewhat larger than the rest, have no anterior communica- 

 tion and receive tributaries from the abdominal muscles and the diaphragm. In the 

 middle portion of their course the upper six or seven veins give off branches, the 

 costo-axillary veins, which ascend towards the axilla and open into either the long 

 thoracic or the thoraco-epigastric vein and so into the axillary, and, as it approaches 

 the vertebral column behind, each vein receives a dorsal branch (ramus dorsalis) which 

 accompanies the spinal branch of the intercostal artery and returns the blood from the 

 skin and muscles of the back and also from the spinal column and its contents, this 

 latter drainage being by means of a spinal branch (ramus spinalis) which connects 

 with the intervertebral veins (page 898). 



Their posterior termination varies considerably in different individuals, especially 

 as regards the upper members of the series. It may be supposed that primarily all 

 the intercostal veins of the right side opened into the azygos vein and all of those of 

 the left side into the hemiazygos or accessory hemiazygos, and this condition holds 

 in the adult with all but the upper two or three veins. On the right side the vein 

 of the first space — that of the second space sometimes uniting with it — frequently 

 accompanies the superior intercostal artery as a right superior intercostal vein, 

 and opens above into either the right innominate or one of its branches, usually the 

 vertebral ; on the left side the vein of the first space opens into the left innominate vein, 

 being sometimes termed the accessory left superior intercostal vein, while the 

 veins of the second, third, and sometimes the fourth spaces unite to a common trunk 

 which crosses the arch of the aorta and opens into the left innominate vein, forming 

 the left superior intercostal vein. It is to be noted that this last is connected 

 with the accessory hemiazygos vein and really represents, in part at least, its upper 

 portion, — a fact which is all the more evident from its frequent connection by means 

 of a fibrous cord with the oblique vein oi the left auricle ; and, furthermore, it may 

 also be pointed out that the veins of the second, third, and sometimes the fourth spaces 

 of the right side usually unite to a common trunk which opens into the azygos 

 vein. 



The principal tributaries and connections of the intercostal veins have already 

 been mentioned, but there remain to be described the interesting arrangement shown 

 by the valves in those veins which connect anteriorly with the internal mammary or 

 musculo-phrenic veins. So far as this arrangement is concerned, each vein may be 

 regarded as consisting of three portions : (i) an anterior portion, in which the con- 

 cavities of the vtlves look towards the internal mammary or musculo-phrenic veins ; 

 (2) a posterior portion, in which the valves look towards the azygos or hemiazygos 

 veins ; and (3) an intermediate portion, which is destitute of valves. As a result of 

 this arrangement the blood of the anterior portion of each vein must pass to the internal 

 mammary veins (page 860), that of the posterior portion to the azygos or hemiazygos, 

 while in the intermediate portion it may pass in either direction. Bij^t it is with this 

 intermediate portion that the costo-axillary veins are connected, so that in the upper 

 six or seven veins, in addition to passing partly anteriorly and partly posteriorly, some 

 of the blood takes an ascending direction and empties into the axillary vein. 



