DIGESTIVE SYSTEM.] of the plantar surface of the tarsus, is especially large in Armadillos. There is also usually a pair of sesamoid bones on the plantar aspect of each metatarso-phalangeal articula tion. The metatarsal bones never exceed five in number, and the phalanges follow the same numerical rule as in the manus, never exceeding three in each digit. Moreover, the first digit, counting from the tibial side, or hallux, resembles the pollex of the hand in always having one segment less than the other digits. As the function of the hind foot is more restricted than that of the hand, the modifications of its structure are less striking. In the C etacea and the Sirenia it is entirely wanting, though in some members of the first-named order rudiments of the bones of the first and second segment of the limb have been detected. DIGESTIVE SYSTEM. The search after the purpose which every modification of structure subserves in the economy is always full of interest, and, if conducted with due caution and sufficient knowledge of all the attendant circumstances, may lead to important generalizations. It must always be borne in mind, however, that adaptation to its special function is not the only cause of the particular form or structure of an organ, but that this form, having in all probability been arrived at by the successive and gradual modification of some other different form from which it is now to a greater or less degree removed, has other factors besides use to be taken into account. In no case is this principle so well seen as in that of the organs of digestion. These may be con sidered as machines which have to operate upon alimentary substances in very different conditions of mechanical and chemical combination, and to reduce them in every case to the same or precisely similar materials; and we might well imagine that the apparatus required to produce flesh and blood out of coarse fibrous vegetable substances would be different from that which had to produce exactly the same results out of ready-made flesh or blood ; and in a very broad sense we find that this is so. If we take a large number of carnivorous animals, belonging to different funda mental types, and a large number of herbivorous animals, and strike a kind of average of each, we shall find that there is, pervading the first group, a general style, if we may use the expression, of the alimentary organs, different from that of the others. There is a specially carnivorous and a specially herbivorous modification of these parts. But, if function were the only element which has guided such modification, it might be inferred that, as one form must be supposed to be best adapted and most perfect in its relation to a particular kind of diet, that form would be found in all the animals consuming that diet. But this is far from being the case. The Horse and the Ox, for instance, two animals whose food in the natural state i precisely similar, are yet most different as regards the structure of their alimentary canal, and the processes involved in the preparation of that food. Again, the Sea and the Porpoise, both purely fish-eaters, which seize anc swallow and digest precisely the same kind of prey in pre cisely the same manner, have a totally different arrange ment of the alimentary canal. If the Seal s stomach is adapted in the best conceivable manner for the purpose i has to fulfil, why is not the Porpoise s stomach an exac facsimile of it, and vice versa 1 We can only answer, thi Seal and Porpoise belong to different natural groups o animals, formed on different primitive types, or descendec from differently constructed ancestors. On this principl only can we account for the fact that, whereas, owing t the comparatively small variety of the different alimentarj substances met with in nature, few modifications woul 361 ppear necessary in the organs of digestion, there is really ndless variety in the parts devoted to this purpose. The digestive apparatus of mammals, as in other vertebrates, consists mainly of a tube with an aperture jlaced at or near either extremity of the body, the oral and the anal orifice, with muscular walls, the fibres of which are so arranged as by their regular alternate con- raction and relaxation to drive onwards the contents of The he tube from the first to the last of these apertures. The mouth, interior or commencing portion of this tube and the parts around it are greatly and variously modified in relation to .he functions assigned to them of selecting and seizing the ood, and preparing it by various mechanical and chemical processes for the true digestion which it has afterwards to undergo before it can be assimilated into the system. For this end it is dilated into a chamber or cavity called the mouth, bordered externally by the lips, usually muscular and prehensile, and supported by a movable framework which carries the teeth, organs the structure and modi fications of which have been already described. The roof of the mouth is formed by the palate, terminating behind by a muscular, contractile arch, having in Man and some few other species a median projection called uvula, beneath which the mouth communicates with the pharynx. The anterior part of the palate is composed of mucous membrane tightly stretched over the flat or slightly concave bony lamina which separates the mouth from the nasal passages, and is generally raised into a series of transverse ridges, which sometimes, as in Ruminants, attain a con- iderable development. In the floor of the mouth, between the rami of the mandible, and supported behind by the hyoidean apparatus, lies the tongue, an organ the free surface of which, especially in its posterior part, is devoted to the sense of taste, but which also by its great mobility, being composed almost entirely of muscular fibres, performs important mechanical functions connected with masticating and procuring food. Its modifications of form in different mammals are very numerous. Between the long, extensile, vermiform tongue of the Anteaters, which is essential to the peculiar mode of feeding of those animals, and the short, sessile, and almost functionless tongue of the Porpoise, every intermediate condition is found. Whatever the form, the upper surface is always covered with numerous fine papillae, in which the terminal filaments of the gustatory nerve are distributed. In connexion with the buccal cavity is an extensive and Salmir complex glandular apparatus which pours its secretions into glands, it secretions which constitute the fluid commonly known as saliva. This apparatus consists of small glands em bedded in the mucous membrane or submucous tissue lining the cavity of the mouth, and which are of two kinds (the follicular and the racemose), and of others in which the secreting structure is aggregated in distinct masses removed some distance from the cavity, other tissues besides the lining membrane being usually interposed, and pouring their secretion into the cavity by a distinct tube or duct, which traverses the mucous membrane. To the latter alone the name of " salivary glands " is ordinarily appropriated, although the distinction between them and the smaller racemose glands is only one of convenience for descriptive purposes, their structure being more or less identical ; and, as the fluids secreted by all become mixed in the mouth, their functions are, at all events in great part, common. Under the name of salivary glands are commonly included (1) the "parotid," situated very superficially on the side of the head, below or around the cartilaginous external auditory meatus, and the secretion of which enters the mouth by a duct (often called Steno s or Stenson s) which crosses the masseter muscle and opens into the upper and back part of the cheek ; and (2) the " submaxillary," situ-
XV. - ,| 6