336 DISCOVERY REPORTS 



two further lobes of vascular network, apparently isolated from the main body, were 

 found in the larger foetus (Fig. i). In the smaller they appeared to be joined to the 

 main mass of the rete, forming a continuous band of the retial mass extending along the 

 heads of the ribs as far as the 6th dorsal vertebra. In adult whales also these last two 

 lobes appear to be isolated, so that the rete may be said to extend pretty continuously 

 from the ist cervical to the 6th dorsal vertebra. 



RELATION TO BLOOD VESSELS 



The blood leaves the heart by the great aortic arch, which curves over the root of the 

 left lung as described by Turner (Fig. 3). It gives off on its course the left common 

 carotid and the left subclavian arteries from the summit of its curve. 



Shortly before the origin of the left common carotid, midway between the two lobes 

 of the thymus gland (removed in Fig. 3), the brachiocephalic or innominate artery 

 runs forward and outwards from the root of the systemic towards the right side of the 

 body. Slightly lateral to the thymus it splits into the right common carotid and sub- 

 clavian arteries, as in other mammals. On the left, as has been mentioned, these take 

 origin from the systemic arch itself. On each side the subclavian continues forwards 

 and outwards, running, in the proximal part of its course, ventral to the pleural dome, 

 and distally over the ventral face of the scalene muscle. In contact with the inner face 

 of the first rib the subclavian divides into a brachial artery, passing to the flipper, and 

 an internal mammary artery, passing backwards across the middle of the ribs along the 

 inner face of the side wall of the thorax. This artery supplies, by means of its numerous 

 branches, the more distal portions of the intercostal spaces. The brachial artery is very 

 small in comparison with the subclavian and internal mammary arteries. 



During its course beneath the pleural dome the subclavian artery gives off a stout 

 branch — the posterior thoracic (Figs. 1,3,4 A), which runs round the anterior face of 

 the pleural dome and backwards along the roof of the thoracic cavity underneath the 

 pleuron; it is in contact with the ventral surfaces of the heads of the first five ribs, dis- 

 appearing dorsally between the transverse processes of the 5th and 6th dorsal vertebrae. 

 This artery forms a ridge at the extreme anterior face of the dome, causing a crescentic 

 groove on the forepart of the lung where that organ presses against it. In its position 

 and relations this posterior thoracic artery corresponds to a much enlarged superior 

 intercostal artery, such as is found in other mammals. 



The posterior thoracic artery is accompanied mesially by a stout complementary 

 posterior thoracic vein which emerges from between the 7th and 8th dorsal transverse 

 processes (Figs. 1,3,46). This vein passes rostrally with regard to the corresponding 

 artery on the inner faces of the ist and 2nd ribs, and is joined by the internal mammary 

 vein. Blood returns from the flipper as a large brachial vein, which is joined by numerous 

 plexiform veins from the ventral face of the scalene muscle between that muscle and 

 the sterno-mastoid. This network forms an "axillary venous plexus" filling practically 

 the whole of the space between these two muscles in the area of the axilla. It may, 



