The digestive apparatus (of which the teeth may be considered a portion) acquires its greatest completeness and elaboration in this group. The leading differences w hich it presents, and which depend mainly on the nature of the food, have been already noticed in the article DIGESTION.
The organs of circulation and respiration require no special remark, as, in all essential points, they closely resemble the corresponding organs in man. See CIRCULATION and RESPIRATION.
The kidneys of mammals generally agree with those of man in their internal struc ture. See KIDNEYS. In some animals (especially those that live in water), they arc much lobulated. In the ox, there are 20 free rounded lobules; in the bear, 40 or 50; in the seal, 70 or 100; while in the true cetacea, the separate lobules are so numerous as to give a racemiform appearance to the kidney. All mammals are provided with a urinary bladder, in which the excretion may accumulate so as only to require being discharged at intervals. This organ is largest in the herbivora, and very small in the cetacea.
The nervous system is remarkable for the large size of the brain, and especially of its hemispheres, in comparison with the rest of the nervous system. The surface of the cerebral hemispheres exhibits a more or less convoluted appearance, the number of the convolu tions being to a gre-at degree in correspondence with the amount of intelligence of the animal. The hemispheres are united at their lower parts (except in the implacental mam mals) by a fibrous band or corm-I-fissure, termed the corpus eallosum, which does not occur in the other vertebrates. In the lowest mammals, the cerebellum is situated quite behind the hemispheres, so as to be visible from above; as we get higher in the scale, it is more •sr less covered, in consequence of the prolongation pf the hemispheres backwards; until in the highest apes and in man it is almost completely concealed.
The organs of the senses are constructed on precisely the same plan as in man. The most important variations are noticed in the articles EAR, EYE, etc.
The muscular system. generally accords with that of man, but presents many remarkable deviations, according to the form of the skeleton, the use of the several organs in the act
.of locomotion, the natural posture of the animal, etc.
From the structural characteristics and peouliarities of mammals, we turn to that class of animals in their relations to man.
The uses to which mammals are subsemient are almost innumerable, and will readily suggest themselves.
Mammals are very generally distributed over the surface of the globe. The mammalia 'of certain regions evince very decided peculimities of structure and distribution, as is well .exemplified by the case of the two lowest or implacental orders—the monotremata and marsupialia, both of which (with the sole exception of the opossum, one of the marsupi .alia, in America) are confined to the Australian province. Many other facts of equal interest in the distribution of mammals will be readily ascertained in 3Ir. A. R. Wallace's Geographical Distrzbution (1876).
The subdivision nf the mammals into these orders closely approximates to that of -Cuvier, as may be seen by a reference to the following table of his sub-classes and orders of the mammalia: This classification is given in the present article because, although imperfect in many respects (for example, in placing the sloth above the horse, the bat above the dog, and 'the hedgehog above the elephant), it has been retained in a large number of popular works. In consequence of these obvious imperfections, subsequent attempts at new .classifications have been made by several of the most eminent zoologists, some of whom, as Waterhouse and Owen, have taken the brain, and others, as Milne-Edwards, Gervais, and -Vogt, the placenta, as the basis of classification. Our limited space forbids us from discussing the merits of these systems. The grounds on which prof. Owen bases his cerebral classification may be found in his essay On, the Classification and Geographical Distribution of the Mammalia, 1859; while the arguments in favor of the,placental classi .fication may be found in prof. Huxley's lectures on classification, published in the Medical Times for the year 1863.