106 Mammalia. Circulatory System. 



Artiodactyla. The Portal vein shows valves in some Euminants. 

 Cete. The non- valvular structure of the veins is remarkable. 



External circular fibres. 



Sectorialia. The hepatic veins in the Seal have an outer coat of 

 circular fibres. Wagner remarks that a peculiar annular muscle, 

 about an inch in breadth is met with in the Seals on the trunk of 

 the inferior cava above the diaphragm and venous sac, which can 

 cut off the return of blood to the heart. 



A right and a left vena cava superior are found in 



some Insectivor a f e.g. Hedgehog; some Cheiroptera, e.g. 'Bat; most 

 Eodentia, e.g. llabbit, Squirrel, Rat, Mouse ; in the Proloscidia ; 

 and in all Marsupialia and Monotremata. 



A right vena cava superior and a left azygos venous trunlCf prevails 

 in most of the larger quadrupeds : it is found in 



some Insectivora, e.g. Talpa ; some Eodentia, e.g. Cavia; some 

 Perissodactyla, ^.^. Horse; and some Artiodactyla, e.^. the Omnivorous 

 Suidce, and many Euminantia Cavicornua. 



A right vena cava superior and a left cardiac venous trunh or coronary 

 sinus, prevails in the higher Mammalia, 



e.g. in Bimana; many Quadrumana ; most Bigitigradej Semi- 

 plantigrade, and Pinnigrade Sectorialia ; and nearly all Cete. 



The Inferior cava is more or less dilated 



in some Eodentia, e.g. Castoridcd; 



in some Sectorialia, e.g. Lutra and Phoca (in the latter thi8 

 dilatation reaches to the greatest extent ; it occurs close to, and al- 

 most within the liver, and extends to the diaphragm, above which 

 the vein is again of the common width) : 



in the Belphinidce (Cete) and the Monotrematous Ornithorhyncus. 



Venous plexuses. 



Cete. The veins at the back of the Thoracic-abdominal cavity 

 have a characteristic expansion and plexiform multiplication. The 

 chief abdominal reservoir is formed by the vast psoadic* plexus 

 which extends from behind the hinder end of the kidney to the 

 hinder end of the abdomen : in the Porpoise it forms a mass of 

 reticulate veins upwards of an inch in thickness. The caudal vein 



* So called, as being situated between the psoas muscle and tbe peritoneum. 



