4 MAMMALS. 



With the exception of the word Beasts, we have no true English term for this 

 group of animals. The term Quadrupeds was, indeed, long in popular use, but 

 since it is inapplicable to whales, while it would also include most Reptiles, it is 

 now largely superseded by the term Mammals, derived from the most obvious 

 peculiarity of the class. 

 Characteristics ^ n Addition to the presence in the females of mammary glands 



of the secreting the milk, by means of which the young are nourished, 

 Mammals. Mammals differ from the other higher Vertebrates by the mode in 

 which the lower jaw is articulated to the skull. Thus in other Vertebrates this 

 articulation is effected by the intervention of a separate squared bone, known as the 

 quadrate, upon the lower end of which the articular hollow of the lower jaw plays,, 

 while its upper end is articulated to the skull proper. In Mammals, however, this 

 intermediate bone is absent, and the lower jaw consequently articulates by means 

 of a convex surface, or condyle, directly with the walls of the skull itself. More- 

 over, in all Mammals, each half of the lower jawbone consists of but a single bone > 

 instead of several distinct bones joined together. Thus an isolated jawbone is 

 always sufficient to prove whether its owner was a Mammal or some other 

 Vertebrate. Another very important feature of Mammals is that they always have 

 hair (although it may be only a few bristles on the mouth) on some portions of 

 their bodies during a certain period of their existence. Again, that portion of 

 the large cavity of the body which contains the heart and lungs is completely 

 separated by a horizontal partition, known as the midriff or diaphragm, from the 

 one containing the stomach and intestines. Moreover, at least in all living 

 members of the class, the brain of Mammals is much more highly organised than 

 that of other animals; one of its distinctive features being the presence of a 

 transverse band on its lower surface, by means of which its two lateral halves 

 are intimately connected together. 



The above are a few of the chief features distinguishing Mammals from all 

 other Vertebrates, but we may now briefly notice some in which they differ from 

 certain of the lower classes, although agreeing with others. One of the most 

 important of these differences is that the skull of Mammals is jointed to the first 

 vertebra by means of a pair of transversely disposed bosses, or condyles, as they are 

 technically called. In this respect Mammals are broadly distinguished from Birds 

 and Reptiles, in which there is but a single condyle, placed in the middle line of 

 the skull. Frogs and newts, constituting the class of Amphibians, agree, however, 

 with Mammals in the mode by which the skull is jointed to the backbone ; although 

 they differ from them very widely in other parts of their organisation. 



On the other hand, Mammals differ from Fishes, Amphibians, and 

 Circulation. 



Reptiles, in having warm blood, which is propelled from a four- 

 chambered heart through a double circulatory system; one part of this system 

 causing the blood to pass through the lungs for the purpose of taking in a fresh 

 supply of oxygen from the air, and the other being subservient to the supply of 

 freshly oxygenated blood to the various organs and members of the body. This 

 circulatory system also differs from that of Birds and Reptiles in that the blood 

 for the nourishment of the body is propelled from the heart by a single vessel, 

 known as the aorta, which passes over the left branch of the windpipe ; whereas 



