44 MAMMALIA. 



covering ; each of these is continued into a tortuous cystic duct, which 

 unites with three ducts from the liver into a common excretory 

 canal. 



The Spleen is always present, but varies in form and size in the, 

 several orders. Thus it is, in general, elongated and small in the 

 Ruminantia, Carnivora, and Makis ; short, broad, and flat, in the 

 Apes ; largest, in relation to the liver, in Man. The Cetacea here 

 also exhibit a striking anomaly, the Dolphins having from 5 to 6 les- 

 ser spleens, lying near to the larger one, which is always proportion- 

 ally but slightly developed. In Man also there occurs in rare cases 

 an abnormal subdivision of this organ. 



The Pancreas is, for the most part, formed of two, and rarely of 

 three, principal lobes. It has one or two excretory ducts, which 

 last number occurs also not unfrequently in Man. When it is sim- 

 ple, as in all the Apes, the Ruminantia, and most Carnivora and Ro- 

 dentia, it usually falls, as has been mentioned above, into the 

 biliary duct, but if a second one is present, as in the Horse, Hog, 

 Otter, and Beaver, it enters by itself farther behind into the duo- 

 denum. 



ORGANS OF CIRCULATION. 



THE Heart consists, as in Man and Birds, of two perfectly dis- 

 tinct auriculo-ventricular chambers. It is surrounded by a peri- 

 cardium, the lower part of which is not generally united with the 

 diaphragm, as is the case in Man and the Orang, and is frequently, 

 as in the Hedgehog, remarkably thin and delicate. The form of 

 the heart is, in general, more rounded and not so elongated as in 

 Man. In the Cetacea it is very broad and flat. In the herbivo- 

 rous Cetacea (Halicore and Manatus) the heart is cleft in a pecu- 

 liar manner, the division into two ventricles being indicated exter- 

 nally by a deep fissure in its apex. The foramen ovale is, as in 

 Man, always closed, and only open as an accidental abnormal con- 

 dition. The internal arrangement of the muscles and valves exhib- 

 its several trifling varieties : thus the Eustachian valve is wanting 

 in many genera, while on the contrary in the Elephant it is very 

 large and spirally twisted. In the Ornithorynchus the fleshy condi- 

 tion of the valves (valv. tricuspidales) in the right heart reminds us 

 of that in Birds. In some herbivorous Mammalia, as in the Ox, 

 Sheep, Hog, and Goat, there is found as a normal formation, in the 

 septum ventriculorum, below the origin of the aorta, a cruciform 



