68 AZYGOS VEIN 



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 in 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 Echidna 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 ; in 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 Kodents 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, i.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 Trichosurus vulpecula. In 

 Halmaturus bennettii there are two azygos veins, one left and one 

 right, of which the left is rather the larger. 1 



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. 136. 



