54 AN INTRODUCTION TO ZOOLOGY 



unable to pass through the membrane in this manner, and are termed 

 colloids. 



We have already seen that the alimentary canal is a complicated 

 tube running from mouth to anus, but nowhere opening into the 

 body itself. The food then, in order to be utilised by the animal, 

 must find its way into the tissues through the walls of the canal, but 

 these walls consist of living cells, and hence form an organic mem- 

 brane. Some constituents of food are insoluble, and of the re- 

 mainder only the mineral salts and certain of the carbohydrates are 

 crystalloids, while the remainder of the carbohydrates, the fats, and 

 the proteins are colloids. The problem confronting an animal when 

 its food is secured therefore is, how can the insoluble substances 

 be made soluble, and how can the colloids in solution be converted 

 into crystalloids so as to pass through the gut wall ? In order to 

 effect the necessary changes, we find in the higher animals a complex 

 system of glands has been evolved, and the food is subjected to 

 the action of a number of different substances produced by them. 

 The various alterations undergone by the food up till the time it 

 passes into the wall of the canal are all included in the term digestion. 



It is beyond our scope to enter into the details of the chemical 

 changes brought about by the enzymes in the digestive juices, but 

 a consideration of a few of the more important will serve to illustrate 

 their general mode of action. The principal ones act by Hydrolysis, 

 i.e. they cause the molecules of the substance to decompose or 

 undergo cleavage by a reaction with water.* They may be separated 

 into protein-splitting enzymes, starch-splitting enzymes, fat- 

 splitting enzymes, sugar-splitting enzymes, and so on, according to 

 the material upon which they act. 



Digestion commences in the mouth by the action of the ptyalin 

 of the saliva attacking and splitting up the starchy food. It is 

 continued in the stomach where, in addition to encountering the 

 hydrochloric acid, it is acted upon by the two ferments, pepsin and 

 rennin, which are present in the gastric juice. The former acts 

 upon proteins, breaking them down to simpler substances, peptones, 

 and the latter coagulates milk. The food now, after mastication 

 and partial digestion in the stomach, assumes a batter-like con- 

 sistency and gives an acid reaction to litmus. In this condition it 

 is termed chyme. After it has been thoroughly acted upon by the 

 gastric juice it is passed on to the duodenum, and here comes under 

 the influence of the bile and pancreatic juice. 



* Thus, for example, one carbohydrate, maltose, is transformed into another, 

 dextrose, by the action of the enzyme maltase 



C 12 H 22 O n +H 2 0=C 6 H 12 6 +C b H 12 6 . 



(maltose) (dextrose) (dextrose) 



