Royal Society. SS9 



The right anterior primitive vein in all cases persists as the right 

 or ordinary vena cava superior ; but the left vein either remains un- 

 occluded, and returns the blood from the left side of the head and 

 neck, from the left upper limb, the left side of the thorax, and from 

 the substance of the heart; or, owing to a partial occlusion, returns 

 only the blood from the left side of the thorax and from the substance 

 of the heart ; or, owing to still further occlusion, from the substance 

 of the heart alone. Hence three principal groups arise. 



a. In the first group a right and a left superior vena cava exist, 

 connected by a cross branch at the root of the neck, as in the Mono- 

 tremata, Marsupialia, the Elephant, most Rodentia, the Hedgehog 

 and the Bat. 



b. In another group a right superior cava and a left vena azygos 

 exist, as in the Sheep, Goat, Ox, Pig, Horse, Mole and Guinea Pig. 



c. In the third group there is found, besides the right vena cava 

 superior, only a left cardiac venous trunk or coronary sinus, together 

 with the vestiges already described, as in the Cetacea, Carnivora and 

 Quadrumana, as well as in Man. 



In each of these groups subordinate varieties are shown and clas- 

 sified. 



3. The almost numberless varieties of the great anterior veins in the 

 human subject are then arranged on principles similar to those adopted 

 in regard to the different conditions found among Mammalia ; but 

 the groups are arranged in the inverse order, and the usual con- 

 dition of the veins in Man is included as a necessary element in the 

 series. 



In one large class of cases, comprehending three groups similar to 

 those of the different Mammalia already defined, the cross branch in 

 the neck is always present. 



g,. In the first group there is a right vena cava superior, and a left 

 cardiac venous trunk or coronary sinus. This is the ordinary con- 

 dition. Further subdivisions arise, depending on peculiarities of the 

 vena cava itself, which are rare; of the azygos system, which are 

 exceedingly numerous; and of the coronary vein and sinus, which 

 are again uncommon. Transposition occasionally produces a further 

 modification, in which the superior cava is found on the left side; 

 whilst the coronary sinus, the oblique vein and the vestigial fold of 

 the pericardium, exist on the right. 



b. Ill another group there might exist a right vena cava superior 

 and a left vena azygos, as in the Sheep ; but no example of this pos- 

 sible variety has yet been met with in the human subject. 



c. In the third group a right and a left superior cava coexist, as 

 in the Elephant, constituting what is termed a double vena cava su- 

 perior. Thirty examples of this condition are adduced, of which 

 eleven only have occurred in adult and otherwise perfect hearts. 

 One of these was met with by the author, and is specially described. 



Lastly, a separate or second class consists of those cases in which 

 the cross branch is wanting, and which are, accordingly, destitute of 

 the characteristic mammalian type, and present, as in Birds, the per- 

 sistent condition of four independent lateral venous trunks. 



The paper is illustrated by original drawings, of the development 



