68 AZYGOS VEIN CHAP. 
cavities of the mammal’s heart, which fourfold division it shares 
with birds alone, do not exactly correspond compartment for 
compartment with those of the bird’s heart, at least im so far as 
concerns the ventricles. For the reptilian heart is provided with 
only one ventricle, and therefore the division of that cavity 
must have been independently accomplished in mammals and 
in birds. 
There are two features in the venous system which distinguish 
all the Mammalia (with the exception of Hehidna in one of 
these points) from vertebrates standing lower in the series. The 
hepatic portal system is limited to a vein which conveys to the 
liver blood derived from the alimentary tract; m no mammal 
except in Echidna is there any representative of the anterior 
abdominal vein of lower vertebrates. In that animal there is 
such a vein, which apparently arises from a capillary network 
upon the bladder and passes up, supported by a membrane, along 
the ventral wall of the abdomen to the liver, thus emptying 
blood into that organ exactly as does the anterior abdominal 
vein of the frog. In no mammal is there any trace of a renal 
portal system. The kidneys derive their blood from the renal 
arteries only. 
Many mammals have two superior venae cavae; this is the 
case, for instance, in the Elephant and the Rodents and other 
types lying comparatively far down in the series. In most if 
not in all mammals there are considerable remains of one of the 
posterior cardinals, in the form of the azygos vein, which opens 
into the vena cava superior or pre-caval vein, @.e. the superior 
cardinal just before the latter debouches into the heart. This 
one posterior cardinal is usually on the right side; but it may 
be on the left side, for instance in 77richosurus vulpecula. In 
Halmaturus bennettii there are two azygos veins, one left and one 
right, of which the left is rather the larger.’ 
Urinary Organs.—The kidneys in the Mammalia have a 
compact form, which contrasts with the somewhat diffuse and 
vaguely-outlined kidneys of the Sauropsida. In mammals the 
organ is as a rule of that peculiar shape which is called “ kidney- 
shaped”; a depression termed the hilum, which receives the 
ducts of the glands, indenting the border of an otherwise oval- 
shaped gland. In some few mammals the kidney is broken up 
1 Beddard, Proc. Zool. Soc. 1895, p. 186. 
