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now largely superseded by the term Mammals, derived from the most obvious 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