THE THORACIC RETE 34S 



definition of any distinct azygos or hemiazygos trunk is obscured by the characteristic 

 expansion and plexiform multiplication of the veins at the back of the thoracic cavity. 

 Schulte (p. 467) quotes von Baer as stating that the azygos is missing altogether in the 

 Porpoise, while in the Sei Whale Schulte (pp. 467-8) describes it as present rudi- 

 mentarily. He mentioned a small venous trunk having a zig-zag course above the 

 ganglionated chord on the right side, receiving laterally branches from the right inter- 

 costal spaces and mesially branches from the left intercostals. Possibly the small vessel 

 described by Schulte corresponds to the right-hand member of the pair found by me. 



It is suggested that this pair of small veins and the single trunk by which it joins the 

 left brachiocephalic are the homologues of the azygos veins. There is considerable 

 variation in the disposition of the azygos throughout the Mammalia (Owen, vol. in, 

 P- 555)- Among the Marsupials both azygos veins are present in the primitive Saurian 

 post-cardinal position. In the Ungulates there is a single (right) azygos entering the 

 single (right) precava. In the Hog the single left azygos enters the right auricle 

 directly. In the Antelope two azygos veins are present — the left being the larger (Owen, 

 quoting Hunter, vol. ill, p. 555). In the Rhinoceros the right precava receives the right 

 azygos vein while the left drains into the left subclavian. That the azygos of the Fin 

 Whale, then, should lie on the left and enter the left brachiocephalic (innominate of 

 other mammals) is not an unusual feature. 



It may be said, therefore, that the descending intraspinous veins (Figs. 4 B and 6) 

 and the intercostal veins of the 6th space and all those behind it enter the azygos venous 

 system. They are complementary to the corresponding ascending and intercostal 

 arteries which are derived from the dorsal aorta. 



In the 7th intercostal space, however, the main intercostal vein joins the posterior 

 thoracic as well as the azygos, to which small cross connections also run (Figs. 4 B 

 and 6). This is also observed in the 6th space, so that these two spaces would appear to 

 be transitional between the segmental and posterior thoracic intercostal systems. 



In the 5th space veins exactly complementary to the arteries drain into the posterior 

 thoracic. In the 4th and 3rd spaces the intercostal veins also empty into the posterior 

 thoracic system, but may show slight variations. For example, in the larger foetus the 

 veins from these two spaces on the left side joined together to form a common trunk 

 before reaching the posterior thoracic. In the smaller foetus they passed straight into 

 that vein on both sides of the body. 



The 1st and 2nd spaces are, as already stated, covered by the rete. 

 From the above description and from the figures it will be seen that the intercostal 

 blood supply is derived from four sources : 



(i) The internal mammary, supplying the middle and distal parts of all the spaces 

 except the first. 



(ii) The segmental system, supplying the 6th space and all behind it. 

 (iii) The posterior thoracic system, supplying the 3rd~5th space, 

 (iv) The rete mirabile, which overlies the whole of the 1st and the proximal part of 

 the 2nd space. 



3-2 



