The mammary g/and exists in both sexes, but except in very rare cases, it is only in the female that they secrete milk. Their number is never less than two, and when more, is generally nearly proportional to that of the young produced at each birth. In the monkey, the elephant, the goat, the mare, etc., there are two; in the cow, stag, and lion, four; in the cat, eight; in the rabbit, ten; in the pi,„0., generally twelve; and in the rat ten or twelve. These glands are often blended together, as in the cow; and their number is then indicated externally by that of the nipples or teats. Their position also varies; in the monkeys and bats, and in the herbivorous cetacea, they are situated on the thorax, as in man; in most of the carnivorous animals they are situated on the abdomen as well a.s on the thorax; while in the mare, cow, sheep, etc., they are placed still further back, aear the hip-joint The skin in the greater number of mammalia is covered with hair, a form of tegumen tary appendage peculiarly characteristic of this class. In the cetacea, however, we have an almost entire absence of hair; one of its uses—that of keeping the heat within the body—being here provided for by the thickening of the skin and the deposition of the blubber beneath it. In the edentate, the hairy covering is almost entirely replaced by horny scales, as in the scaly ant-eater, or by bony plates, as in the armadillo. In the quills of the porcupine and the spiny bristles of the hedgehog, we have other modifications of hair which depart less from its ordinary character than those just mentioned. Moreover, the claws, nails, and hoofs of all mammals, the horn or horns on the nose of the rhinoceros, and the horns of the hollow-horned ruminants (such as the ox, sheep, etc.), are all composed of a substance which is only a modification of hair.
The skeleton, as a, general rule, governs the general form of the body. In its general conformation, it shows a close analogy with that of man, which is described in the article SKELETON; the differences which are remarked among the various animals of this class mainly depending (1) upon the absence of posterior limbs in the marine mammals, such as the dugong, the porpoise, and the -Whale; (2) upon the diminished nuniber of digits (see HAND and Fool), and upon the absence of the clavicle in the greater number of those species whose anterior limbs serve only for motion; (8) upon variations in the number of vertebrm; and (4) upon the inequalities in the relative sizes of the same bones (Milne Edwards).
Although the same bones enter into the formation of every mammalian skull, great differences present themselves in different skulls, according as the face is more or less prolonged, or, on the other hand, the brain-ease or cranium is more or less developed. In proportion as a mammal is removed in classification from man, we find that the cranium is diminished; that the face is prolonged by extension of the jaws and nasal cavities; that the orbits are directed outwards, and are less distinct from the temporal fossm; and that the occipital foramen (through which the spinal cord passes) and the condyles (by which the head articulates with the first vertebra of the spinal column) are placed towards the posterior part of the skull, instead of occupying the middle of its inferior surface, as in man. Among the most characteristic points in the mammalian
skull generally may be mentioned (1) that the lower jaw articulates directly with the skull, there being no intervening tympanic bone, such as occurs in the other vertebrates; and (2) that the occipital bone of the skull articulates with the first vertebra by two con dyles, one on either side of the occipital foramen, instead of by a single condyle, as in the other vertebrates.
The vertebral column, except in relation to its length, closely resembles that in man, where there are 7 cervical, 12 dorsal, 5 lumbar, 5 sacral, and 5 caudal vertebrm. The cervical vertebres are almost universally 7 in number, however long or short the neck may be, the only known exceptions being two cetaceans (manetus and rytina), which have 6, and the three-toed sloth, which has 9. The number of dorsal vertebres ranges from 11 to 23, which latter number occurs in the two-toed sloth. The lumbar vertebree range from 2 to 9, the most common number being 5. The sacral vertebra3, which coalesce to form the sacrum and to support the pelvic arch, vary from 2 (in the monotremata and mar supialia) to 6 (in the mole), the most common number being 4. In the cetacea, the rudi mentary pelvis is loosely connected with a single vertebra, and there cannot be said to be a sacrum. The caudal vertebra3, which in man and the higher apes coalesce to form the coccyx, are usually very numerous, 20 or 80 being a common number, and 40 occurring in the long-tailed ant-eater. The form and number of caudal verte,bres vary in accordance with the purposes to which the tall is applied; and the special uses of this organ are numerous. For example, in the kangaroo it serves as a third leg when the animal stands erect; in the American monkeys, and in some of the opossums, it is a prehensible organ; and in the cetacea and in the beaver it is a powerful instrument of propulsion in water. The ram correspond in number to the dorsal vertebrm, and, as a general rule (excepting in the monotremata), they are connected superiorly not only with the bodies of two ver tebrm, but with the transverse process of one of them, and hence present corresponding articular surfaces. The sternum is generally divided into three portions; the middle one, in place of being represented by a single piece, as in man, usually consisting of as many pieces as there are true ribs. It is very short in the cetaeea, and is very long in the car nivore and edentate, extending in some cases nearly to the pelvis. In certain cases, in which it is necessary that the anterior members. should be endowed with unusual strength, as in the bats, moles, and armadillos, there is a projecting keel upon this bone (as in birds) for the attachment of powerful pectoral muscles.