The last and highest class of the Vertebrata, that of the Mammalia, may be Shortly defined as including Vertebrate animals in which some part or other of the integument is always provided with hairs at some time of life ; and the young are nourished, for a longer or shorter time, by means of a special fluid - the milk - secreted by special glands - the mammary glands. These two characters are of themselves sufficient broadly to separate the Mammals from all other classes of the Vertebrate sub-kingdom. In addition, however, to these two leading peculiarities, the Mammals exhibit the following other characters of scarcely less importance:

1. The skull articulates with the vertebral column by means of a double articulation, the occipital bone carrying two condyles, in place of the single condyle of the Reptiles and Birds.

2. The lower jaw or mandible consists of two halves or rami, united anteriorly by a symphysis, but not necessarily anchylosed; but these are each composed of a single piece, instead of being complex and consisting of several pieces, as in the Reptiles and Birds. Further, the lower jaw always articulates directly with the squamosal element of the skull, and is never united to an os quadratum, as in the Sauropsida.

3. The two hemispheres of the cerebral mass, or brain proper, are united together by a more or less extensively developed "corpus callosum" or commissure.

4. The heart consists - as in Birds - of four cavities or chambers, two auricles and two ventricles. The right and left sides of the heart are completely separated from one another, and there is no communication between the pulmonary and systemic circulations. The red blood - corpuscles are non-nucleated, and, with the exception of those of the Camelidae, they are circular biconcave discs. There is only one aorta - the left - which turns over the left bronchus. and not over the right, as it does in Birds.

5. The cavities of the thorax and abdomen are completely separated from one another by a muscular partition - the diaphragm or midriff.

6. The respiratory organs are in the form of two lungs placed in the thorax, but none of the bronchi end in air-receptacles, distributed through the body, as in Birds.

7. The embryo mammal is invariably enveloped in an amnion, and an allantois is never wanting. The allantois, however, either disappears at an early period of life, or it develops the structure known as the "placenta." The placenta is a vascular organ which serves as a means of communication between the parent and the foetus, but it will be noticed more particularly hereafter.

8. In no Mammal do the visceral arches and clefts of the embryo ever carry branchiae, as they do in the fishes and Amphibians.

These are the essential characters which distinguish the Mammalia as a class, but it will be necessary to consider these, and some other points, in a more detailed manner.

In the first place, with regard to the osteology of the Mammals, the following points should be noticed :

With the exception of the Whales and Dolphins (Cetacea), and the Dugongs and Manatees (Sirenia), the vertebral column is divisible into the same regions as in man - namely, into a cervical, dorsal, lumbar, sacral, and caudal or coccygeal region (see fig. 241). In the Cetacea and Sirenia the dorsal region of the spine is followed by a number of vertebrae which compose the hinder extremity of the body, but which cannot be separated into lumbar, sacral, and caudal vertebrae.

In spite of the great difference which is observable in the length of the neck in different Mammals, the number of vertebrae in the cervical region is extraordinarily constant, being almost invariably seven, as in man. In this respect there is no difference between the Whale and the Giraffe. The only exceptions to this law are the Manatees (Manatus) which have but six cervical vertebrae ; the three-toed Sloth (Bradypus tridactylus), which is commonly regarded as possessing nine, though competent anatomists would refer the posterior two of these to the dorsal region; and one of the two-toed Sloths (Choloepas Hoffmanni), which has only six cervical vertebrae.

The dorsal vertebrae are mostly thirteen in number, but they vary from ten to twenty-four. In man there are twelve, in one of the Armadillos only ten, and in the two-toed Sloths and the Hyrax the maximum is attained. The lumbar vertebrae are usually six or seven in number, rarely fewer than four. In Man they are five in number, and they are reduced to two in the two-toed Sloth, one of the Ant-eaters, and the Duck-mole.

The first vertebra, or atlas, always bears two articular cavities for the reception of the two condyles of the occipital bone; and the second vertebra, or axis, usually has an "odontoid" process, on which the head rotates. In the true Whales, however, in which the cervical vertebrae are anchylosed together to a greater or less extent, and the neck is immovable, the odontoid process is also wanting.

In almost all Mammals the spinous processes of the dorsal vertebrae are very largely developed for the attachment of the structure which is known as the ligamentum nucha. This is a great band of elastic fibrous tissue, which is attached in front to the occipital bone and spinous processes of the cervical vertebrae, and which relieves the muscles of the task of supporting the head in those Mammals which progress with the body in a horizontal position. The development of the ligamentum nucha is consequently, as a rule, proportionate to the size of the head and the length of the neck. In Whales no such apparatus is necessary, owing to the fixation of the cervical vertebrae by anchylosis; and in Man, who walks erect, the ligamentum nuchae can hardly be said to exist as a distinct structure, being merely represented by a band of fascia.

The number of lumbar and sacral vertebrae, as we have seen, varies in different Mammals; but ordinarily some of the vertebrae are anchylosed into a single bone, and have the iliac bones abutting against them, thus constituting the "sacrum" of human anatomists. In the Celacea and Sirenia, in which the hind-limbs are wanting, and the pelvis rudimentary, there is no "sacrum."