In front, above and below, are four chisel-shaped teeth, known as the incisors. Next to these on either side is a tooth longer and thicker than the incisors, called the canine. Back of these are two short, rounded and double pointed teeth, the bicuspids, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the molars (B and C, Fig. 65). Since the molar farthest back in each jaw is usually not cut until maturity, it is called a wisdom tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length.
Fig. 66—Diagram showing directions of muscular fibers in tongue.
2. The Tongue.—The tongue is a muscular organ whose fibers extend through it in several directions (Fig. 66). Its structure adapts it to a variety of movements. During mastication the tongue transfers the food from one part of the mouth to another, and, with the aid of the cheeks, holds the food between the rows of teeth. (By an outward pressure from the tongue and an inward pressure from the cheek the food is kept between the grinding surfaces.) The tongue has functions in addition to these and is a most useful organ.
[pg 144]3. The Muscles of Mastication.—These are attached to the lower jaw and bring about its different movements. The masseter muscles, which are the heavy muscles in the cheeks, and the temporal muscles, located in the region of the temples, raise the lower jaw and supply the force for grinding the food. Small muscles situated below the chin depress the jaw and open the mouth.
Fig. 67—Salivary glands and the ducts connecting them with the mouth.
4. The Salivary Glands.—These glands are situated in the tissues surrounding the mouth, and communicate with it by means of ducts (Fig. 67). They secrete the saliva. The salivary glands are six in number and are arranged in three pairs. The largest, called the parotid glands, lie, one on either side, in front of and below the ears. A duct from each gland passes forward along the cheek until it opens in the interior of the mouth, opposite the second molar tooth in the upper jaw. Next in size to the parotids are the submaxillary glands. These are located, one on either side, just below and in front of the triangular bend in the lower jaw. The smallest of the salivary glands are the sublingual. They are situated in the floor of the mouth, on either side, at the front and base of the tongue. Ducts from the submaxillary and sublingual glands open into the mouth below the tip of the tongue.
The Saliva and its Uses.—The saliva is a transparent and somewhat slimy liquid which is slightly alkaline. It[pg 145] consists chiefly of water (about 99 per cent), but in this are dissolved certain salts and an active chemical agent, or enzyme, called ptyalin, which acts on the starch. The ptyalin changes starch into a form of sugar (maltose), while the water in the saliva dissolves the soluble portions of the food. In addition to this the saliva moistens and lubricates the food which it does not dissolve, and prepares it in this way for its passage to the stomach. The last is considered the most important use of the saliva, and dry substances, such as crackers, which require a considerable amount of this liquid, cannot be eaten rapidly without choking. Slow mastication favors the secretion and action of the saliva.
Deglutition.—Deglutition, or swallowing, is the process by which food is transferred from the mouth to the stomach. Though this is not, strictly speaking, a digestive process, it is, nevertheless, necessary for the further digestion of the food. Mastication and insalivation, which are largely mechanical, prepare the food for certain chemical processes by which it is dissolved. The first of these occurs in the stomach and to this organ the food is transferred from the mouth. The chief organs concerned in deglutition are the tongue, the pharynx, and the esophagus.