CHAPTER XXIX 

 ALIMENTARY CANAL 



" I receive the general food at first, 

 \\'hich you do live upon ; and fit it is, 

 Because I am the store-house, and the shop 



Of the whole body : 



Though all at once cannot 



See what I deliver out to each : 

 Yet I can make my audit up, that all 

 From me do back receive the flour of all, 

 And leave me but the bran." 



Shakespeare. 



As has been indicated (Chap. XXII.) the non-gaseous imports 

 are submitted to a certain amoimt of manufacture before being 

 handed over to the inland transport service for transportation 

 to the cells of the body. For instance, proteins have to be split 

 into their constituent amino-acids, carbohydrates are broken 

 down into monosaccharides, and fats undergo some change. 

 In addition to these changes in molecular complexity and pre- 

 ceding them come, in many cases, changes of physical state. 

 Most of oiu' foodstuffs are solid or semi-solid, and in such a state 

 are useless to the organism. Before they can be split into their 

 constituent imits they must be rendered soluble. 



General. In brief, the f miction of the alimentary canal is to 

 provide (1) a series of mills and factories where food may be 

 comminuted and dissolved in water, (2) a series of factories for 

 breaking down the dissolved foods into units which the organism 

 is capable of absorbing, (3) a mechanism for absorbing these units, 

 (-t) a mechanism for eliminating the waste material, (5) a means 

 of transport from one factory to another, and (6) an adequate 

 control over these various processes so that all may be co-ordinated. 



In structure, the alimentary canal is a tube passing longitudi- 

 nally through the body, having anteriorly a voluntary mechanism 

 for receiving and grinding food ; intermediately, stations, not 

 controlled by the will, for completely breaking down the food 

 mass to convenient units and for absorbing the same, and pos- 

 teriorly, a semi-voluntary mechanism for ejection of waste. 



I. The mouth is the port of the alimentary transport system. 

 First, by nose and eye the cargo is sighted and its nature estimated. 

 Messages are sent inland, factories get busy and all is ready when 



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