CHAPTEE XII 

 DIGESTION 



THE great majority of the materials taken in as food by animals require treat- 

 ment of some kind in order to enable them to be carried by the blood or other 

 fluids to the various organs requiring them. 



In the green plant, the food-stuffs do not require treatment of this kind, but, 

 even here, stored products such as starch and protein require the action of certain 

 enzymes before they are again available for the use of the cells, or to be conveyed 

 to distant growing parts. 



In the animal, the conversion of food-stuffs into diffusible or assimilable 

 substances is known as digestion, and is carried on in the alimentary canal, chiefly 

 by means of enzymes secreted into the cavity by the various glands opening into 

 it or situated in its walls. In the digestion of certain food-stuffs, especially that 

 of cellulose in the herbivora, bacteria play an important part. 



To describe the great variety of digestive mechanisms met with in the animal 

 kingdom would take far more space than is permissible here. It may be said in 

 general that the object of these mechanisms is to ensure the effective action of the 

 digestive enzymes, and the due absorption by the blood or lymph of the products 

 of their activity. Details of these mechanisms may be found in the article by 

 Biedermann (1911). 



INTRACELLULAR DIGESTION 



In the unicellular organisms the whole process takes place within the one cell. 

 The food-stuffs are taken in, a vacuole containing liquid is formed around them, 

 and the necessary enzymes secreted into this vacuole. Material undigested is 

 extruded through any part of the cell. Although raw starch appears difficult of 

 attack by protozoa, the fact that boiled starch is hydrolysed shows that they 

 possess an amylase. They also store glycogen, a fact indicating the reversible 

 action of an amylase. But, in any case, 'protein appears to be their chief food, 

 obtained in the main from bacteria and algte. According to the work of 

 Nierenstein (1905), the reaction of the contents of the food vacuole is at first acid 

 to neutral red, and subsequently becomes alkaline. During the acid period, no 

 digestion takes place, but the reaction seems to be connected with the killing of 

 the bacteria taken as food. The actual digestion occurs during the alkaline stage. 

 Mouton (1902) prepared an enzyme from a large number of amoeba-, and found 

 that it would not attack living bacteria, but dead ones rapidly, so that the 

 preliminary killing in the acid stage is of importance. The enzyme acted best in 

 a medium alkaline to litmus, but acid to phenolphthalein, that is, faintly alkaline ; 

 it produced tyrosine and is, accordingly, to be considered as a trypsin. 



Phagocytosis. We find the process of iritracellular digestion, similar to that of 

 the amoeba described above, still present in the amoeboid cells of the higher 

 multicellular organisms. The leucocytes of vertebrates require special attention. 

 In these, Metschnikoff has shown that the taking up of living bacteria plays an 

 important part in the defence against infection by micro-organisms. He called the 

 process " phagocytosis." It is not confined to certain leucocytes of the blood, but 

 is manifested by large cells found in the peritoneal cavity and elsewhere. The 

 reader may consult the book by Metschnikoff (1901) for further details. The 

 process of phagocytosis has been referred to previously (page 3) and Ledingham's 

 work mentioned. According to this observer, the bacteria in the blood are not 



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