150 
MR. MARSHALL ON THE DEVELOPMENT 
Group A. A right and a left vena cava superior. — This condition exists in a large 
number of the lower Mammalia, viz. in the Monotremata* and Marsupialia-f" ; in 
most Rodentia, as in the Dormouse*, Marmotte*, Rat:|;§, Echimys*, Mouse:J:§, 
Squirrel*:!:, Beaver*, Hamster*, Mole of the Cape*, Hare:|: and Rabbit*§:J:. It is 
found also in the Elephant* || amongst Pachydermata ; and in the Hedgehog:!:^ and 
Bat*:|:§ amongst Insectivora and Cheiroptera. 
In all such cases, the cross branch in the neck, when sought for, has been found. 
The left vena cava superior always descends in front of the root of the left lung, and 
then turns beneath the base of the heart, and after receiving the great coronary and 
other cardiac veins in its course, opens into the right auricle^. 
Owing to suhordinate modifications in the azygos veins, this group may be again 
subdivided as follows : — 
a. An azygos vein on each side. 
a. Of equal size. 
Ex. Monotremes. Marsupials!?). 
b. Of unequal size. 
a. Left azygos the larger. 
Ex. Hedgehog**, Rat, Mouse**. 
(3. Right azygos the larger. 
Ex. Rabbit. 
b. An azygos vein on one side only. 
a. A left azygos only. 
Ex. (?). 
h. A right azygos only. 
Ex. The Squirrel. The Hare. The Rabbit is a near approach to this 
condition, the left azygos being very insignificant. 
c. Azygos vein wanting (?). 
Group B. A right vena cava superior and a left azygos venous trunk . — This arrange-^ 
ment prevails in most of the larger quadrupeds. It occurs in the Ungulates, Rumi- 
Meckel (Anat. Comp, par Jourdan, t. ix.) is the authority for including these animals. 
\ Meckel. Also Owen (Cycl. Anat. and Phys.). 
I Rathke (Dritter Bericht, &c. Konigsberg, 1838). 
§ The Author. || Mus. Anat. of University Coll, and of Royal Coll, of Surg. London. 
^ In the Marsupialia, and also in the Monotremata, the left upper cava joins the inferior cava just before that 
vein expands into the right auricle (Owen, Articles Marsup. and Monofrem. Cycl. Anat. and Phys., vol. iii. 
pp. 307, 309 and 390). This peculiarity, which is particularly marked in those marsupials which have a large 
vena cava inferior (owing to the size of their hinder limbs), appears to be due merely to an opening out, as it 
were, of the orifices of the two veins , so that they meet and blend with each other. 
** Eustachius (Opuscula Anat. de vena sine pari, p. 273) describes the large left azygos of the Hedgehog 
and Mouse, and also a splitting of the inferior cava, in the former animal, into two branches, of which one is 
evidently the left upper cava. 
