THE CIRCULATORY SYSTEM 185 



early stage (eighteen to twenty-one days after fertilisation), the 

 post-caval vein never develops, and the posterior cardinals take 

 its place. 



In one such case, described by Kollmann, the two posterior cardinal 

 veins persisted to the level of the third lumbar vertebra. At the crura of 

 the diaphragm, within the aortic foramen, the right cardinal vein was con- 

 nected with the left by three branches. The trunk thus related lay to the 

 left of the aorta, and ran on as a persistent portion of the left cardinal. At 

 the level of the tenth thoracic vertebra the vessel turned to the right, and 

 after this it was the right cardinal vein which was continued to its point of 

 entrance into the vena cava superior. The ductus venosus Arantii was absent ; 

 and the circulation in the liver remained entirely embryonic, the hepatic 

 veins still entering the heart separately. This remarkable case was that of 

 a man of twenty-eight, who had committed suicide. 



In Man, and certain Mammals (Apes, Lemurs, Carnivora, 

 Whales, and Edentates), the left vena cava superior early degener- 

 ates and disappears, with the exception of its basal portion, which 

 remains as " the coronary sinus," so-called on account of its 

 receiving the intrinsic cardiac veins. [The great veins of the head, 

 neck, and fore-limb on the left side become connected with those 

 of the right by a transverse trunk, derived from the left innomin- 

 ate vein the two innominate or brachio-cephalic veins uniting 

 to form the single " superior cava."] In this we have to deal 

 with the modification of a condition which in other Mammals 

 (Eodents, Insectivora, Bats, and Ungulates) is retained throughout 

 life ; [and it is an interesting circumstance that among these a 

 transverse connection between the great veins of the neck strongly 

 suggestive of that above described may not infrequently be estab- 

 lished (ex. Lepus), without any accompanying reduction of the 

 left pre-caval.] 



The venous system, so rich in variations, is well known to 

 possess valves which prevent regurgitation, [and thus ensure the 

 maintenance of the single circle of the circulation.] In keeping 

 with this we should expect to find such valves chiefly in the 

 limbs, i.e. where the venous stream I refer especially to the lower 

 limbs already has great difficulties to overcome. This expecta- 

 tion is fulfilled ; but when we reflect that the ancestor of Man 

 himself had a quadrupedal ancestry, it follows that there must 

 have been a time in which his thoracic, and abdominal, and dorsal 

 surfaces, now disposed antero-posteriorly, were turned downwards 

 and upwards and were disposed ventro-dorsally. Circulation within 

 the intercostal and lumbar veins must then have been placed under 

 much less favourable conditions than at present ; it had to be 



