INTRODUCTORY 3 
exceptions, circular in outline ; in the lungs being freely suspended 
in a thoracic cavity, separated from the abdomen by a complete 
muscular partition—the diaphragm—which is the principal agent 
in inflating the lungs in respiration ; in having but one aortic arch, 
which curves over the left bronchus ; in the skin being more or less 
clothed with hair; in the greater perfection of the commissural 
system of the cerebral hemispheres, which has either a complete 
corpus callosum, or an incomplete one associated with a very 
large anterior commissure; in having no syrinx or inferior vocal 
organ, but a complete larynx at the upper end of the trachea; 
in having a mandible of which each ramus (except in very early 
developmental conditions) consists of a single bone on each side, 
articulating to the squamosal without the intervention of a quad- 
rate bone; in having a pair of laterally placed occipital condyles 
instead of one median one; and in the very obvious character of 
the female being provided with mammary glands, by the secretion 
of which the young (usually produced alive, although in the lowest 
forms by means of externally hatched eggs) are nourished for some 
time after birth. 
In common with all vertebrated animals, mammals never have 
more than two pairs of limbs ; as the larger number live ordinarily 
on the surface of the earth, in the great majority of the class 
both pairs are well-developed and functional, and adapted for terres- 
trial progression. Mammals are, however, by no means limited to 
this situation. Thus some species spend the greater part of their 
lives beneath the surface, their fore limbs being specially modified 
for burrowing; others, again, are habitually arboreal, their limbs 
being fitted for climbing or hanging to boughs of trees; some are 
as aerial as birds, the fore limbs being developed into wings of a 
special character ; while in others which are as aquatic as fishes, 
the limbs assume the form of fins or paddles. In many of the 
latter the hinder extremities are either completely suppressed, or 
present only in a rudimentary state. In no known mammal are 
the fore limbs absent. 
The hinder extremity of the axis of the body is usually prolonged 
into a tail, which may be a mere pendent appendage, or may be 
modified to perform various functions, as grasping boughs in 
climbing, or even gathering food, in the case of the prehensile- 
tailed Monkeys and Opossums, swimming in the Cetacea, and acting 
as a flap to drive away troublesome insects from the skin in the 
Ungulata. 
The state of development of the young at the time of birth 
varies greatly in the different groups. Thus among the Marsupials 
where there is no connection during intra-uterine life between the 
circulatory systems of the parent and the foetus, the young are 
born in an exceedingly imperfectly developed condition. For their 
