Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/371

Rh DENTAL SYSTEM.] MAMMALIA 349 glands of Antelopes, the preputial glands of the Musk Deer and Beaver (both so well known for the use made of their powerfully odorous secretion in medicine and perfumery) and also of the Swine and Hare, the anal glands of Car- nivora, the perineal gland of the Civet (also of commercial value), the caudal glands of the Fox and Goat, the gland on the humeral membrane of Bats of the geuus Saccopteryx, the post-digital gland of the Rhinoceros, the inter-digital glands of the Sheep and many Ruminants, and numerous others. In some of these cases the glands are peculiar to, or more largely developed in, the male ; in others they are found equally developed in both sexes. DENTAL SYSTEM. The dental system of mammals may be considered rather more in detail than space permits for some other portions of their structure, both on account of the important part it plays in the economy of the animals of this class, and of its interest to zoologists as an aid in classification and identi fication of species. Owing to the imperishable nature of their tissues, teeth are preserved for an indefinite time, and in the case of extinct species often offer the only indications available from which to derive an idea of the characters, affinities, and habits of the animal to which they have belonged. Hence even their smallest modifications have received great attention from comparative anatomists, and they have formed the subject of many special mono graphs. 1 Teeth are present in nearly all mammals, and are applied to various purposes. They are, however, mainly subserv ient to the function of alimentation, being used either in procuring food by seizing and killing living prey or gather ing and biting off portions of vegetable material, and more indirectly in tearing or cutting through the hard protective coverings of food substances, as the husks and shells of nuts, or in pounding, crushing, or otherwise mechanically dividing the solid materials before swallowing, so as to prepare them for digestion in the stomach. Certain teeth are also in many animals most efficient weapons of offence and defence, and for this purpose alone, quite irrespective of subserviency to the digestive process, are they developed in the male sex of many herbivorous animals, in the females of which they are absent or rudimentary. Teeth belong essentially to the tegumentary or dermal system of organs, and, as is well seen in the lower vertebrates, pass by almost insensible gradations into the hardened spines and scutes formed upon the integument covering the outer surface of the body, but in mammals they are more specialized in structure and limited in locality. In this class they are developed only in the gums or nbro-mucous membrane covering the alveolar borders of the upper and lower jaw or the premaxillary and maxillary bones and the mandible. In the process of development, for the purpose of giving them that support which is needful for the per formance of their functions, they almost always become implanted in the bone, the osseous tissue growing up and moulding itself around the lengthening root of the tooth, so that ultimately they become apparently parts of the skeleton. In no mammal, however, does ankylosis or bony union between the tooth and jaw normally take place, as in many fishes and reptiles, a vascular layer of connective tissue, the alveolo-dental membrane, always intervening. 2 1 L. F. E. Rousseau, Anatomie comparee du Systeme Dentaire chez fllomme ct chez Us principaux Animaux,-2d ed. 1839; F. Cuvier, Des Dents ties Mammiferes considerees comme caractHres zoologiques, 1822- 25 ; R. Owen, Odontography, 1840-45 ; C. G. Giebel, Odonto- graphie, 1855; C. S. Tomes, Manual of Dental Anatomy, Human and Comparative, 2d ed., 1882. 2 The lower incisors of some species of Shrews are, however, said to become ankylosed to the jaw in adult age. The presence of two or more roots, frequently met with in mammals, implanted into corresponding distinct sockets of the jaw, is peculiar to animals of this class. The greater number of mammalian teeth when fully Struc- formed are not simple and homogeneous in structure, tu re of but are composed of several distinct tissues. teeth. 1. The jimfy), a soft substance, consisting of a very delicate Pulp, gelatinous connective tissue, in which numerous cells are imbedded, and abundantly supplied with blood-vessels and nerves, constitutes the central axis of all the basal part of the tooth, and affords the means by which the vitality of the whole is preserved. The nerves which pass into the pulp and endow the tooth with sensibility are branches of the fifth pair of cranial nerves. The pulp occupies a larger relative space, and performs a more important purpose in the young growing tooth than afterwards, as by the calci fication and conversion of its outer layers the principal hard constituent of the tooth, the dentine, is formed. In teeth which have ceased to grow the pulp occupies a com paratively small space, which in the dried tooth is called the pulp cavity. This communicates with the external surface of the tooth by a small aperture at the apex of the root, through which the branches of blood-vessels and nerves, by which the tooth receives its nutrition and sensi tiveness, pass in to be distributed in the pulp. In grow ing teeth the pulp cavity is widely open below, while in advanced age it often becomes obliterated, and the pulp itself entirely converted into bone-like material. 2. The dentine or ivory forms the principal constituent of Dentine, the greater number of teeth. When developed in its most characteristic form, it is a very hard but elastic substance, white, with a yellowish tinge, and slightly translucent. It consists of an organic matrix, something like but not identical with that of bone, richly impregnated with calcareous salts (chiefly phosphate of lime), these constitut ing in a fresh human tooth 72 per cent, of its weight. When subjected to microscopical examination it is seen to be everywhere permeated by nearly parallel branching tubes which run, in a slightly curving or wavy manner, in a general direction from the centre towards the free surface of the tooth. These tubes communicate by open mouths with the pulp cavity, and terminate usually near the periphery of the dentine, by closed ends or loops, though in Marsupials and certain other mammals they penetrate into the enamel. They are occupied in the living tooth by soft gelatinous fibrils connected with the cells of the pulp. A variety of dentine, permeated by canals con taining blood-vessels, met with commonly in fishes and in some few mammals, as the Megatherium, is called vaso- dentine. Other modifications of this tissue occasionally met with are called osteo-dentine and secondary dentine, the latter being a dentine of irregular structure which often fills up the pulp cavity of old animals. 3. The enamel constitutes a thin investing layer, com- Enamel, plete or partial, of the outer or exposed and working surface of the dentine of the crown of the teeth of most mammals. This is the hardest tissue met with in the animal body, containing from 95 to 97 per cent, of mineral substances (chiefly phosphate and some carbonate of lime, with traces of fluoride of calcium). Its ultimate structure consists of prismatic fibres, placed generally with their long axes at right angles to the free surface of the tooth. Enamel is easily distinguished from dentine with the naked eye, by its clear, bluish-white, translucent appearance. 4. The cementum or crusta petrosa is always the most Ce- externally placed of the hard tissues of which teeth are composed, as will be understood when the mode of develop ment of these organs is considered. It is often only found as a thin layer upon the surface of the root, but sometimes, as in the complex-crowned molar teeth of the Horse and