THE THORACIC RETE 343 



spaces, as do the superior intercostal arteries and veins of other mammals. It will be con- 

 venient to give a brief description of the blood supply to the intercostal areas generally. 



The middle and distal parts of the intercostal musculature are served by numerous 

 branches of the internal mammary system. Mention has already been made of these 

 and they need not be further described. 



All the intercostal areas behind and including the 6th (that is, behind the 6th rib) 

 are supplied from the dorsal aorta by means of segmental arteries (Figs. 4 A and 6). 

 Each of these passes outwards, ventral to the intervertebral ligaments, and splits 

 into an ascending branch to the dorsal musculature, serial with those coming off 

 from the posterior thoracic, and a main intercostal branch to the intercostal muscles 

 (Figs. 4 A and 6g, d). In front of the 6th intercostal space the supply is derived not 

 from the dorsal aorta but from the posterior thoracic artery (Fig. 6 b), which gives off 

 a double series on each side. Each of the 3rd, 4th and 5th intercostal spaces is served 

 by a branch from the posterior thoracic artery, serial with those from the aorta, and by 

 a smaller branch serving their proximal parts. The 1st and 2nd intercostal spaces are 

 not supplied with blood by means of arteries, either from the dorsal aorta or from the 

 posterior thoracic, but are covered by the main body of the rete mirabile, which appears 

 to be in intimate contact with the musculature underneath the pleuron. The 1st space 

 is completely covered by the rete, and receives no branches from the internal mammary 

 system, while the 2nd space is covered for about its proximal third and receives distally 

 the first of the internal mammary series. 



The intercostal venous return may broadly be said to be complementary to the 

 arterial supply. The 8th space and all those behind it are drained by pairs of segmental 

 veins. The 3rd, 4th and 5th drain into the posterior thoracic vein, while the 6th and 7th 

 must be regarded as transitional since they contribute to both systems (Figs. 4 B and 6). 



Behind the 7th space — that is in the 8th and all behind it — the intercostal veins and 

 the descending intraspinous veins on each side drain into a small vein which runs ventral 

 to the bodies of the vertebrae and arches over the segmental arteries at their origins 

 from the aorta. These small veins run mesial to the ganglionated chords on each side. 

 Just in front of the 6th dorsal vertebra the right member of this pair of venous trunks 

 crosses the posterior insertion of the rectus capitis anticus muscle and joins the left- 

 hand member of the pair at the level of the 3rd dorsal vertebra. The resulting single 

 trunk enters the brachiocephalic vein above the root of the left lung (Fig. 6). 



It has been stated by all authors that the azygos and hemiazygos veins of other 

 mammals are absent or very much reduced in the Cetacea. Breschet and Bouvier were 

 both of the opinion that the neural venous sinuses take the place of the azygos system. 

 In the account given by the former author (pp. 18-19, P^ 4> %• J ) trie l ar g e descending 

 intraspinous vein between the 3rd and 4th rib was described and figured on the left 

 side only, while that on the right was omitted, so that the neural venous sinuses were 

 stated by Breschet to present an azygous condition. Bouvier noted that the neural 

 venous sinuses were not azygous, but nevertheless considered them analogous in 

 function to the azygos vein. Owen (vol. in, p. 553) stated that in the Porpoise the 



