According to Owen, the typical permanent dentition of a diphyodont Mammal would be expressed by the following formula:
1 - 1
1 - 1
The four kinds of teeth are indicated in such a formula by the letters - incisors i, canines c, praemolars pm, molars m. The numbers in the upper line indicate the teeth in the upper jaw, those in the lower line stand for those in the lower jaw; and the number of teeth on each side of the jaw is indicated by the short dashes between the figures.
As regards the digestive system of the Mammalia, salivary glands are present in all except the true Cetacea. The alimentary canal has in most cases essentially the same structure as in man; and the same accessory glands are present - namely, the liver and pancreas. Some very remarkable modifications occur in the structure of the stomach and in the termination of the intestine; but these will be noticed in speaking of the orders in which they occur. The cavity of the abdomen is always separated from that of the thorax by a complete muscular partition - the diaphragm - as is the case in no other Vertebrate animals. The abdomen contains the greater portion of the alimentary canal, the liver, spleen, pancreas, kidneys, and other organs. The thorax mainly holds the heart and lungs.
The heart is contained in a serous bag, the pericardium, and consists (as in Birds) of two auricles and two ventricles. The effete and deoxygenated blood is returned from the tissues by the veins, and is conducted by the two venae cavae to the right side of the heart into the right auricle. From the right auricle it passes into the right ventricle, whence it is propelled through the pulmonary artery to the lungs. Having been submitted to the action of the air, the blood, now arterialised, is carried by the pulmonary veins to the left auricle, and thence into the left ventricle. From the left ventricle the aerated blood is driven through the aorta and systemic vessels to all parts of the body. In Mammals, therefore, as in Birds, the pulmonary and systemic circulations are altogether distinct and separate from one another. The two sides of the heart - except in the foetus and as an abnormality in adults - have no communication with one another except by means of the capillaries.
The lungs of Mammals differ from those of Birds in being freely suspended in the thoracic cavity, the greater part of which they fill, and in being enclosed freely in a serous sac (pleura) which envelops each lung. The lungs are minutely cellular throughout, and the bronchi never open on the surface of the lung into a series of air-receptacles communicating with one another, and placed in different parts of the body, as is the case in Birds.
There is no "inferior larynx," in any Mammal, and the upper aperture of the true larynx is always protected by an epiglottis.
The kidneys in Mammals are situated in the lumbar region, and exhibit a division of their substance into cortical and medullary portions.
There are two ovaries in the Mammals, and the oviducts are known as the "Fallopian tubes." Each oviduct dilates on its way to the surface into a uterine cavity, which opens into the vagina. In the Monotremes and Marsupials this primitive condition is retained throughout life, the uterus remaining double, and opening by two apertures into the cloaca or vagina. In most cases this condition is so far modified in the adult, that the two uteri have coalesced inferiorly, so as to have only a single opening into the vagina, whilst they separate into two horns or "cornua" superiorly. Only in the Monkeys and in Man have the two uteri completely coalesced to form a completely single cavity, into the "fundus" of which the Fallopian tubes open. In male Mammals there are always two testes present. In many Mammals the testes are permanently retained in the abdominal cavity, and there is no scrotum. This is the case in the Monotremes, the Elephants, all the Cetacea, and many of the Edentata. Mostly, however, the testes at an early period of life are transferred from the abdomen to a pouch of integument called the "scrotum." Usually the scrotum is placed beneath the pubic arch and behind the penis, but this position is reversed in the Marsupials.
Mammary glands are present in all Mammals, and they are regarded by Huxley as an extreme modification of the cutaneous sebaceous glands. In the male Mammals the mammary glands are present, but, under all ordinary circumstances, they remain functionally useless and undeveloped. Considerable differences obtain as to the number and position of the mammary glands in different cases: but they are always placed on the inferior surface of the body, and their ducts in the great majority of cases open collectively upon a common elevation - the "teat" or "nipple." In the Monotremata, however, there are no nipples, the ducts of the mammary glands opening either into a pouch of the integument (Echidna) or upon a flat surface (Ornithorhynchus).
The young Mammal is nourished for a longer or shorter time by the milk secreted by the mammary glands of the mother. In ordinary cases the milk is obtained by voluntary suction on the part of the young animal; but in the Marsupials the young are at first unable to suck for themselves, and the milk is forced out of the gland by the contractions of a special muscle.
The nervous system of Mammals is chiefly remarkable for the great proportionate development of the cerebral mass as compared with the size of the spinal cord. In the higher Mammals, again, the hemispheres of the cerebrum are much more largely developed proportionately than the remaining parts of the brain. The brain of the Mammals is chiefly distinguished from that of the lower Vertebrata by the fact that the two hemispheres of the cerebellum are united by a transverse commissure - the pons Varolii; and the hemispheres of the cerebrum are connected by a great commissure - the corpus callosum - which is, however, of small size in the lower Mammalia.
The senses, as a rule, attain great perfection in the Mammals; and the only sense which appears to be ever entirely wanting is that of vision. The sclerotic coat of the eye is never supported by a ring of bony plates as in Birds and many Reptiles. As a rule, in addition to the upper and lower eyelids there is a third perpendicular lid - the membrana nictitans - but this is wanting or quite rudimentary in Man and in the Monkeys.
An external ear or concha for collecting the vibrations of sound is usually present, but is wanting in the Cetacea, many of the Seals, and in some other cases.
The integument is furnished over a greater or less portion of its surface with the epidermic appendages known as " hairs."
These are developed, much as feathers are, upon little eminences or papillae of the dermis, but they do not split up in the process of development as feathers do. In the Mam's or Scaly Ant-eater the epidermic appendages are in the form of horny scales, and not uncommonly they are developed into long spines, as in the Echidna, Porcupine, and Hedgehog. The only apparent exception to the universal presence of hairs in some part or other of the skin of all Mammals is constituted by the Cetacea, the great majority of which are without hairs in the adult state. Some, however, occasionally possess a few bristles in the neighbourhood of the mouth even when fully grown. And the Dolphins, which are totally hairless when adult, exhibit tufts of hair on the muzzle in the foetal state.
The claws, hoofs, and nails of Mammals, and the horny sheaths of the horns of the Cavicorn Ruminants, are also of the nature of epidermic growths.
Lastly, the Armadillos are remarkable for having plates of bone developed in the dermis over a greater or smaller portion of the surface.