CHAP. IX The Divided Labours of the Body 145 



supply of oxygen to the tissues is also a part of these first 

 processes of nutrition. Being a gas, it is treated in a 

 special way which will be described immediately. 



Digestion. — The various food-stuffs have various chemi- 

 cal qualities. After being swallowed they enter a long 

 tube, the digestive tract or alimentary canal. Within 

 this canal they are subjected to the action of various 

 digestive juices prepared by masses of cells called glands. 

 Saliva is one of these juices, gastric juice is another, 

 pancreatic juice is another. The effect of these juices 

 upon the food is that most of it is dissolved in the juice 

 and made diffusible. Thus we see an example of the 

 division of labour. An amoeba flows round a solid particle 

 of food and digests it. In the higher animals the cells of 

 the digestive glands are specialised for this particular func- 

 tion and do little else. 



Absorption. — After the food is digested it leaves the 

 alimentary canal, and is absorbed into the blood-vessels 

 and lymphatics in the walls of the canal. Absorption is not 

 a mere process of diffusion. It is diffusion modified by the 

 cells lining the alimentary tract. Certain chemical changes 

 are effected at the same time. Most of the absorbed food 

 passes to the liver ; but the fat does not go directly into 

 the blood, being first absorbed into that other system of 

 vessels, the lymphatics. Eventually it also gets into the 

 blood ; for the two streams are connected. 



The Worit of the Liver and the Kidneys. — The cells 

 of the liver secrete a juice called bile, which is poured into 

 the alimentary canal. The exact function of this juice is 

 still doubtful. It has a certain use in the digestion of fats, 

 but it is largely an excretion. The stream of food-stuff 

 going' to the liver contains sugar, the result of the digestion 

 of carbohydrates ; albumen, the result of the digestion and 

 absorption of proteids ; and certain waste nitrogenous 

 matters formed during the digestion of proteids. 



The cells of the liver retain the sugar, store it within 

 themselves, in the same sort of way that a potato stores up 

 starch, and give it up gradually to the blood again. So 

 far as is known they do not affect the albumen in any way, 



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