the sub-epithelial tissue surrounding the enamel organ and interposed between the enamel germ and the developing bony jaw), is composed of nucleated cells arranged in a meshwork, the outer or peripheral part being covered with a layer of columnar nucleated cells called odontoblasts. The odontoblasts form the dentine, while the remainder of the papilla forms the tooth-pulp. The method of the formation of the dentine from the odontoblasts is as follows:-The cells elongate at their outer part, and these processes are directly converted into the tubules of dentine (fig. 172). The continued formation of dentine proceeds by the elongation of the odontoblasts, and their subsequent conversion by a process of Fig. 172.-Part of section of developing tooth of a young rat, showing the mode of deposition of the dentine. Highly magnified. a, outer layer of fully formed dentine; b, uncalcified matrix with one or two nodules of calcareous matter near the calcified parts; c, odontoblasts sending processes into the dentine; d, pulp. The section is stained in carmine, which colours the uncalcified matrix but not the calcified part. (E. A. Schäfer.) calcification into dentine tubules. The most recently formed tubules are not immediately calcified. The dentine fibres contained in the tubules are said to be formed from processes of the deeper layer of odontoblasts, which are wedged in between the cells of the superficial layer (fig. 172) which form the tubules only. Since the papillæ are to form the main portion of each tooth, i.e., the dentine, each of them early takes the shape of the crown of the tooth to which it corresponds. As the dentine increases in thickness, the papillæ diminish, and at last when the tooth is cut, only a small amount of the papilla remains as the dental pulp, and is supplied by vessels and nerves which enter at the end of the fang. The shape of the crown of the tooth is taken by the corresponding papilla, and that of the single or double fang by the subsequent constriction below the crown, or by division of the lower part of the papilla. The enamel cap is found later on to consist (fig. 173) of three parts: (a) an inner membrane, composed of a layer of columnar epithelium in contact with the dentine, called enamel cells, and outside of these one or more layers of small polyhedral nucleated cells (stratum intermedium of Hannover); (b) an outer membrane of several layers of epithelium; (c) a middle membrane formed of a matrix of non-vascular, gelatinous tissue, containing a hyaline interstitial substance. The enamel is formed by the enamel cells of the inner membrane, by the elongation of their distal extremities, and the direct conversion of these processes into enamel. The calcification of the enamel processes or prisms takes place first at the periphery, the centre remaining for a time transparent. The cells of the stratum intermedium are used for the regeneration of the enamel cells, but these and the middle membrane after a time disappear. The cells of the outer membrane give origin to the cuticle of the enamel. The cement or crusta petrosa is formed from the tissue of the tooth sac, the Fig. 173.-Vertical transverse section of th dental X 14 structure and function of which are identical with those of the osteogenetic layer of the periosteum. In this manner the first set of teeth, or the milk-teeth, are formed; and each tooth, by degrees developing, presses at length on the wall of the sac enclosing it, and, causing its absorption, is cut, to use a familiar phrase. The temporary or milk-teeth, are speedily replaced by the growth of the permanent teeth, which push their way up from beneath them, absorbing in their progress the whole of the fang of each milk-tooth, and leaving at length only the crown as a mere shell, which is shed to make way for the eruption of the permanent teeth (fig. 165). Each temporary tooth is replaced by a corresponding tooth of the permanant set which is developed from a small sac set by, so to speak, from the sac of the temporary tooth which precedes it, and called the cavity of reserve. MASTICATION. The act of chewing or mastication is performed by the biting and grinding movement of the lower range of teeth against the upper. The simultaneous movements of the tongue and cheeks assist partly by crushing the softer portions of the food against the hard palate and gums, and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out from between them, until they have been sufficiently chewed. Muscles.-The simple up and down, or biting movements of the lower jaw, are performed by the temporal, masseter, and internal pterygoid muscles, the action of which in closing the jaws alternates with that of the digastric and other muscles passing from the os hyoides to the lower jaw, which open them. The grinding or side to side movements of the lower jaw are performed mainly by the external pterygoid muscles, the muscle of one side acting alternately with the other. When both external pterygoids act together, the lower jaw is pulled directly forwards, so that the lower incisor teeth are brought in front of the level of the upper. Temporo-maxillary Fibro-cartilage.-The function of the interarticular fibro-cartilage of the temporo-maxillary joint in mastication is to serve: (1) As an elastic pad to distribute the pressure caused by the exceedingly powerful action of the masticatory muscles. (2) As a joint-surface or socket for the condyle of the lower jaw, when the latter has been partially drawn forward out of the glenoid cavity of the temporal bone by the external pterygoid muscle, some of the fibres of the latter being attached to its front surface, and consequently drawing it forward with the condyle which moves on it. Nervous Mechanism.-The act of mastication is partly voluntary and partly reflex and involuntary. The consideration of such sensori-motor actions will come hereafter (see Chapter on the Nervous System). It will suffice here to state that the afferent nerves chiefly concerned are the sensory branches of the fifth and the glossopharyngeal, and the efferent are the motor branches of the fifth and the ninth (hypoglossal) cerebral nerves. The nerve-centre through which the reflex action occurs, and by which the movements of the various muscles are harmonised, is situated in the medulla oblongata. In so far as mastication is voluntary or mentally perceived, it becomes so under the influence, in addition to the medulla oblongata, of the cerebral hemispheres. INSALIVATION. The act of mastication is much assisted by the saliva which is secreted by the salivary glands in largely increased amount during the process, and the intimate incorporation of which with the food, as it is being chewed, is termed insalivation. The Salivary Glands. The human salivary glands are the parotid, the sub-maxillary, and the sub-lingual, and numerous smaller bodies of similar structure, and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. Structure. The salivary glands are compound tubular glands. They are made up of lobules. Each lobule consists of the branchings of a subdivision of the main duct of the gland, which are generally more or less convoluted towards their extremities, and sometimes, according to some observers, sacculated or pouched. The convoluted or pouched portions form the alveoli, or proper secreting parts of the gland. The alveoli are composed of a basement membrane of flattened cells joined together by processes to produce a fenestrated membrane, the spaces of which are occupied by a homogeneous ground-substance. Within, upon this membrane, which forms the tube, the nucleated salivary secreting cells, of cubical or columnar form, are arranged parallel to one another enclosing a central canal. The granular appearance frequently seen in the salivary cells is due to the very dense network of fibrils which they contain. When isolated, the cells not unfrequently are found to be branched. Connecting the alveoli into lobules is a considerable amount of fibrous connective tissue, which contains both flattened and granular protoplasmic cells, lymph corpuscles, and in some cases fat cells. The lobules are connected to form larger lobules (lobes), in a similar manner. The alveoli pass into the intralobular ducts by a narrowed portion (intercalary), lined with flattened epithelium with elongated nuclei. The intercalary ducts pass into the intralobular ducts by a Fig. 174.-Section of sub-maxillary gland of dog. Showing gland cells, b, and a duct, a, section. (Kölliker.) in narrowed neck, lined with cubical cells with small nuclei. The intralobular duct is larger in size, and is lined with large columnar nucleated cells, the parts of which, towards the lumen of the tube, present a fine longitudinal striation, due to the arrangement of the cell network. It is most marked in the submaxillary gland. The intralobular ducts pass into the larger ducts, and these into the main duct of the gland. As these ducts become larger they acquire an outside coating of connective tissue, and later on some unstriped muscular fibres. The lining of the larger ducts consists of one or more layers of columnar epithelium, the cells of which contain an intracellular network of fibres arranged longitudinally. Varieties. Certain differences in the structure of salivary glands may be observed according as the glands secrete pure saliva, or saliva mixed with mucus, or pure mucus, and therefore the glands have been classified as : (1) True salivary glands (called most unfortunately by some serous glands), e.g., the parotid of man and other animals, and the |