CHAPTEK VIII. 



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 absorption they would be useless for the pur- 

 poses they have to fulfil; almost the whole of the food undergoes various 

 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. Thence it passes into the 

 intestines, where it meets with the bile, the pancreatic juice and the in- 

 testinal juices, all of which exercise an influence upon that portion of the 

 food not absorbed from 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 of faeces by the anus. 



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

 readily seen from the accompanying diagram (Fig. 165). 



The Mouth is the cavity contained between the jaws and inclosed 

 by the cheeks laterally, and by the lips in front; behind it opens into the 

 pharynx by the fauces, and is separated from the nasal cavity by the hard 

 palate in front, and the soft palate behind, which form 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 and lateral 

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

 covered by stratified squamous epithelium, which is continuous in front 

 along the lips with the epithelium of 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 now consider, in detail, the process of digestion, as it takes 

 place in each stage of this journey of the food through the alimentary 

 canal. 



Mastication. The act of chewing or mastication is performed by 



