8 Oct., 1907.] Elements of AiiiviaL Pliysiology. 589 



the phosphate of lime from bones acts as ballast ; in cats the skin or hair 

 or feathers of their victims. In both animals some earth is. generally eaten 

 with their food and this acts in the same way. In omnivores, such as man 

 and the pig, ballast is present in the form of cellulose and woody fibre in 

 the vegetables eaten. Grazing animals eat enormous masses of material 

 which escape digestion and they have special digestive mechanisms to 

 deal with this residue. The coarse fibres in fodders are only feebly 

 digested and even the fine cellulose of vegetable cells only partially so. 

 The silica of plants is also feebly absorbed. If a herbivorous animal 

 be given a fodder poor in fibre or with fibres too finely chopped, digestion 

 is impaired, the contents of the bowel tend tO' become stagnant and putre- 

 faction by bacteria takes place giving rise to poisonous products which 

 are absorbed. This is well exemplified in cows that " lose their cud " 

 when placed on a fodder too rich and digestible. 



4. Organic Acids. — It is doubtful if carnivores require organic acids 

 but omnivores and herbivores certainly do, and grave constitutional dis- 

 turbances follow upon their disuse. Fortunately all the ordinary vege- 

 table foods are rich in these acids or their salts. Fruits are exceptionally 

 rich in organic acids and to this, as well as their ballast content, is 

 due their beneficial action in man and other omnivores. 



5. Flavouking Substances. — If food is unpalatable, no matter 

 how suitable it is in other respects, it is badly digested. The greater 

 the relish with which an animal eats its food the greater is the amount 

 of digestive juices secreted. The palatableness of foods is largelv de- 

 pendent upon the odours or flavours they possess and these are due 

 to chemical bodies which, though they do not come into any of the 

 categories mentioned, are nevertheless essential constituents of normal 

 food. 



7. Enzymes. — Carnivores and omnivores do' not require any enzymes 

 in their food but with herbivores, and especially the ruminants, these 

 are essential ingredients. As we shall see in the following chapter on 

 digestion, one of the first transformations that food undergoes in the 

 alimentary canal, is an autolysis, or self digestion, bv means of enzymes 

 existing in the living cells of plants. Hi-nce it is that fodders which 

 have been steamed or boiled mav, under certain conditions, be not so 

 well digested as the natural plant substances. 



There are other essential constituents of diet but their exact composi- 

 tion has not been worked out, for mere traces of them are apparently 

 sufficient. It is highly probable that the bodv is dependent for its 

 supply of hormones on certain chemical substances which are present 

 in food though only in small amounts. The effects of deprivation of 

 these bodies are onlv' seen in laboratorv experiments. 



Adventitious Constituents of Food. 

 In the food of carnivores there is little that does not come under the 

 headings already given. In the case of omnivores and herbivores vari- 

 able quantities of substances are taken and absorbed which are either 

 harmless and pass out of the body unchanged in the urine or have their 

 harmful properties neutralized in the liver and leave the body as innocuous 

 compounds. Herbivorous animals absorb from their food considerable 

 quantities of substances allied to carbolic acid, and other substances allied 

 to camphor, also sugars that have little or no fuel value. Useless mineral 

 matter (nitrates, silicates and sulphates) may be absorbed in small quan- 

 tities and excreted in the urine. 



