CHAPTER VII. 



DIGESTION. 



THE object of digestion is to prepare the food to supply the waste of 

 the tissues, which we have seen is its proper function in the economy. 

 Few of the articles of diet are taken in the exact condition in which it 

 is possible for them to be absorbed into the system by the blood-vessels 

 and lymphatics, without which they would be useless fcr the purposes 

 they have to fulfil. Almost the whole of the food, therefore, undergoes 

 various digestive changes before it is fit for absorption. Having been 

 received into the mouth, it is subjected to the action of the teeth and 

 tongue, and is mixed with the first of the digestive juices the saliva. It 

 is then swallowed, and, passing through the pharynx and oesophagus into 

 the stomach, is subjected to the action of the gastric juice the second 

 digestive juice. Thence it passes into the intestines, where it meets with 

 the bile, i\iQ pancreatic juice, and the intestinal juices, all of which exer- 

 cise an influence upon the portion of the food not absorbed in the stomach. 

 By this time most of the food is capable of absorption, and the residue 

 of undigested matter leaves the body in the form offices by the anus. 



The course of the food through the alimentary canal of man will be 

 readily seen from the accompanying diagram (Fig. 1G4). 



The Mouth is the cavity contained between the jaws and inclosed 

 by the cheeks laterally, the lips anteriorly; behind it opens into the pha- 

 rynx by the fauces, and is separated from the nasal cavity above, by the 

 hard palate in front, and the soft palate behind, which forms its roof. 

 The tongue forms the lower part or floor. In the jaws are contained 

 the teeth, and when the mouth is shut these form its anterior bounda- 

 ries. The whole of the mouth is lined with mucous membrane, covered 

 with stratified squamous epithelium, which is continuous in front along 

 the lips with the epithelium and the skin, and posteriorly with that of 

 the pharynx. The mucous membrane is provided with numerous glands 

 (small tubular), called mucous glands, and into it open the ducts of the 

 salivary glands, three chief glands on each side. The tongue is not only 

 a prehensile organ, but is also the chief seat of the sense of taste. 



We shall first* of all devote some little space to the consideration of 

 the structure and development of the teeth, and then shall proceed to- 



