CHAPTER XIII 



THE DIGESTION 



361. The Digestive System. The food, in order to be 

 of service to the body, must undergo several changes. 

 Its nutritious portion must be separated from the innutri- 

 tious ; and the former must be made soluble, or in fit 

 condition to be taken up by the blood and carried to the 

 tissues. This process is called digestion, and the assem- 

 blage of organs by which it is carried on is called the 

 digestive system. The tube which forms the receptacle 

 for the food during digestion is called the alimentary 

 canal. In the carnivora, or flesh-eating animals, whose 

 food contains but a small amount of indigestible matter, 

 the alimentary canal is comparatively short, being, when 

 stretched out, only three or four times the length of the 

 body. In the herbivora, or grass-eating animals, the 

 canal is thirty times the length of the body. In the hog, 

 which is omnivorous ("all-eating"), it is ten times the 

 length of the body. If man's trunk only is counted, the 

 canal is twelve times as long; if his height is counted, 

 the canal is six times as long. 



362. The Alimentary Canal has three coats in its walls 

 throughout its whole length. What is each for? The 

 inner coat, or lining, is a delicate epithelial tissue called 

 the mucous membrane. It forms a smooth lining to 

 prevent friction, and secretes a mucus which serves the 

 same purpose. The next coat is the submucous coat ; it 

 is of elastic connective tissue, and serves to toughen 

 and strengthen the wall, and to bind the mucous coat to 



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