no THE HUMAN SPECIES 



inferior. 1 In the gorilla an artery branches off from thearteria 

 femoralis high up in the thigh and extends to the back of the 

 foot. 



A further peculiarity of man is the coccygeal gland situated 

 at the lower extremity of the coccyx. This gland was recog- 

 nised by its discoverer, Luschka, as a rudimentary organ and 

 is now known to represent an arterial plexus (rete mirabile) 

 formed at the extremity of the arteria sacralis, the rudiment of 

 a caudal plexus such as is found in the caudate mammals. 

 Considerable light has been thrown upon the gradual develop- 

 ment of the human venous system by tracing its history up 

 through the progressive stages of the vertebrates. The fishes 

 have four longitudinal venous trunks, two anterior (jugular) and 

 two posterior (cardinal). These four longitudinal trunks occur 

 also in the amphibians and reptiles, but only during the 

 earliest embryonic periods. Further, in the birds two jugular 

 veins, frequently unequal in size, are present for the anterior 

 parts of the body, but, as in the reptiles, these jugular veins 

 unite with the subclavian veins and thus form the vena cava 

 superior. In mammals the conditions were originally very 

 similar, for they apparently also possess the two jugular and 

 cardinal veins, though in man, the apes, the carnivora and 

 cetaceans, owing to a reduction of the trunk of the vena 

 cava superior sinistra, the vena cava superior dextra has be- 

 come the sole venous blood-vessel for the anterior upper part 

 of the body. 



In man and the apes the vena jugularis interna forms the 

 main artery for conducting the blood from the interior of the 

 skull, a peculiarity distinguishing the Primates from all other 

 mammals, their main artery being the vena jugularis externa. 

 The formation of the vena cava inferior must be traced in the 

 same manner before we can decide what is possessed by man 

 in common with the lower animals and what distinguishes him 

 from them. First in the amphibians do the veins of the posterior 

 parts of the body unite in a vena cava inferior and receive the 

 hepatic veins. Proceeding to the reptiles we find renal and 

 hepatic veins forming the trunk of a vena cava inferior, with 

 which are connected the veins of the vertebral column and of 



1 Wiedersheim, loc. cit., p. 204. 



