Structure of Teeth. —Fig. 102 represents the appearance on a cut being carried straight down through a tooth in its socket. In the very centre of the tooth is a cavity—the pulp cavity (1)—which is filled up with the dental pulp, a soft substance containing a rich supply of blood-vessels and nerves. The vessels and nerves enter by a small opening at the point of the fang. In teeth with two or more fangs the cavity is prolonged in the shape of a fine canal down each fang, to a little opening at the point of each. Surrounding the cavity on all sides is the substance that forms the main part of the tooth—the dentine (b). It consists of fine branching tubes embedded in a hard substance. The tubes contain substance continuous with the pulp of the tooth cavity. Dentine is very hard but not brittle, consisting mostly of phate and carbonate of lime. Ivory is the tine of the elephant's tusk. Outside the dentine of the fang is a stance closely bling bone, and called crusta petrosa or cement (c). In fact it is true bone, but wants the Haversian canals (see p. 58). The fang is fixed in its bony socket (e) by means of a dense fibrous brane (d) which rounds the cement as the periosteum does bone. The dentine of the crown of the tooth is not covered by cement but by the enamel (a), which consists of closely set prisms of a densely hard stance, composed mainly of phosphate of lime and other earthy salts and only 3i per cent of animal matter. In young teeth the surface of the enamel is covered by a delicate membrane, which answers to the popular term "the skin of the teeth ". It is worn off in adult teeth.
The Tongue (t, Fig. 101) is a muscular organ, and is covered by the same membrane that lines the rest of the mouth. On the under surface the membrane forms a fold in the middle line, passing between the tongue and the front of the lower jaw. This fold is some times continuous to the tip of the tongue, bind ing it down and interfering with speech, and in infants with sucking. The upper surface is covered with little projections—the papillae of the tongue, which are connected with taste, and will be considered in the section on TASTE.
The Salivary Glands are three in number on each side of the mouth. Their position is shown in Fig. 103. The parotid gland is situ ated on the side of the face in front of the ear ; the submaxillary is placed below and to the inner side of the lower jaw, in front of the angle of the jaw; and the sublingual is on the floor of the month between the tongue and gums. The two sublingual glands are thus near to one another, one on each side of the fold beneath the tongue. All these glands belong to the class called racemose, from their resemblance to a bunch of grapes. They have little channels or ducts, which give off smaller and smaller branches, the smallest branches ending in little pouches or sacs lined with cells, as the stem of the vine gives off smaller stems which end in the pouch of the grape. Groups of the little pouches are bound together by connective tissue, through which blood-vessels pass. Thus the blood stream is brought so
near to the cells of the pouch that they can derive from it whatever ma terials they need for their nour ishment and activity. From the blood the cells derive the raw material which they work up into the sub stance which it is their business to produce. This substance — the saliva—is then conveyed along the small chan nels or ducts till the common duct is reached which carries the fluid into the mouth. Nerves are also freely distributed to the glands, and it has been found that the activity of the gland is largely regulated by the nerves. Fine fila ments of nerves have even been traced to the very cells that line the pouches of the gland. The main duct which conveys away the fluid saliva from the parotid gland (Stenson's duct) opens on the inner surface of the cheek on a level with the crown of the second molar tooth of the upper jaw, where it may be often felt as a slight swelling. The duct of the submaxillary gland (Wharton's duct) opens at the summit of a soft papilla under the tongue. The ducts of the two glands—one of each side—are readily seen on turning up the tip of the tongue. The sublingual glands have a considerable number of ducts opening in the neighbourhood of Wharton's. The purpose of the fluids poured into the mouth from these glands is discussed further on.
The Pharynx is the upper end of the ali mentary canal, and it forms a blind sac above the level of the mouth (ph, Fig. 101). The mouth opens into it, and straight above that opening there are two openings, by means of which the nasal cavity communicates with the pharynx. About the same level as the open ings into the nasal passages are two apertures, one at each side, which are the mouths of the Eustachian tubes, which pass upwards to the cavities of the ears, the cavity of each side being on the inner side of the drum of the ear. By referring to Fig. 101 it will also be seen that the windpipe opens upwards into the pharynx, but that this communication can be shut off by the lid of the windpipe (ep)—the epiglottis—folding down. Thus there are six openings into the pharynx, and the gullet is the direct continuation of it downwards to the stomach. When one opens the mouth widely before a glass, the back wall seen through the narrowing of the faeces is the wall of the pharynx. The mucous membrane of the pharynx is continuous forwards with that of the mouth, upwards with that of the nostrils and tubes leading to the middle ear, and downwards with that of the windpipe and gullet. It is thus that an inflamed and swollen condition of that membrane, which may have begun as a sore throat, may travel into the nose, may impede the passage of air into the Eustachian tubes, blocking them, and so producing deafness, and down into the windpipe, causing irritable throat and coughing. In the membrane is a large number of glands, the excessive secretion and enlargement of which are so troublesome in relaxed and other conditions of the throat.