130. The General Plan of Digestion. It is evident that the digestive organs will be simple or complex, according to the amount of change which is necessary to prepare the food to be taken up by the blood. If the requisite change is slight, the digestive organs will be few, and their structure simple. But if the food is varied and complex in composition, the digestive apparatus will be complex. This condition applies to the food and the digestion of man. Illustration: Fig. 47.—Diagram of the Structure of Secreting Glands. A, simple tubular gland; B, gland with mouth shut and sac formed; C, gland with a coiled tube; D, plan of part of a racemose gland The digestive apparatus of the human body consists of the alimentary canal and tributary organs which, although outside of this canal, communicate with it by ducts. The alimentary canal consists of the mouth, the pharynx, the œsophagus, the stomach, and the intestines. Other digestive organs which are tributary to this canal, and discharge their secretions into it, are the salivary glands,[20] the liver, and the pancreas. The digestive process is subdivided into three steps, which take place in the mouth, in the stomach, and in the intestines. 131. The Mouth. The mouth is the cavity formed by the lips, the cheeks, the palate, and the tongue. Its bony roof is made up of the upper jawbone on each side, and the palate bones behind. This is the _hard palate_, and forms only the front portion of the roof. The continuation of the roof is called the _soft palate_, and is made up of muscular tissue covered with mucous membrane. The mouth continues behind into the throat, the separation between the two being marked by fleshy pillars which arch up from the sides to form the soft palate. In the middle of this arch there hangs from its free edge a little lobe called the uvula. On each side where the pillars begin to arch is an almond-shaped body known as the tonsil. When we take cold, one or both of the tonsils may become inflamed, and so swollen as to obstruct the passage into the throat. The mouth is lined with mucous membrane, which is continuous with that of the throat, œsophagus, stomach, and intestines (Fig. 51). 132. Mastication, or Chewing. The first step of the process of digestion is mastication, the cutting and grinding of the food by the teeth, effected by the vertical and lateral movements of the lower jaw. While the food is thus being crushed, it is moved to and fro by the varied movements of the tongue, that every part of it may be acted upon by the teeth. The advantage of this is obvious. The more finely the food is divided, the more easily will the digestive fluids reach every part of it, and the more thoroughly and speedily will digestion ensue.