102 3fammaUa. Circulatory System. 



are commonly somewhat smaller in other orders. In Mam- 

 malia tlie blood discs are most numerous, and most minute 

 relatively to the bulk of the body, of all Yertebrata. 



CIECULATORY SYSTEM Special. 



1. Ahwrhent system. 



Quadrumana. The mesenteric glands are specially numerous-. 



Sectorialia. The mesenteric glands are aggregated in one mass, 

 known to old anatomists as the Pancreas Asellii. 



TJngulata. The mesenteric glands are numerous. 



2. The Heart. 



The ^pericardium seldom reaches the diaphragm 



in any of the species composing the following orders, vi%. : 

 Insectivora, CheirojJtera, Rodentia^ In-Enamellata^ Marsupialia, and 

 Monotremata. 

 The Heart shows external signs of division 



in some Rodentia. The apex of the heart is sub-bifid in the Hare 

 and Acouchi. 



In some Cete^ e.g. the Cachalots and Whales, the apex is rounded 

 or rather flattened, and sometimes indented. 



In all Sirenia, the outward division of the ventricles indicated in 

 some Cete is carried to an extent very characteristic of the Order ; 

 but in Ehytina and Manatus the cleft is not quite so deep as in the- 

 Heart of Halicore. 



The fossa ovalis. 



Marsupialia. There is no trace of a * fossa ovalis ' nor of an ' annulus 

 ovalis ' ; and the absence of these structures, which are present in the 

 heart of all the Placental Mammalia, relates to the very brief period 

 during which the auricles intercommunicate in the Marsupials, and 

 to the minute size, and in other respects incompletely developed 

 state, at which the young Marsupial animal respires air by the Lungs, 

 and has the mature condition of the pulmonary circulation established. 



