SECTION IX. 



PHYSIOLOGY OF DIGESTION.^ 



In the preceding pages we have learned that the animals' food 

 constitutes that which they eat and drink. We will now discuss 

 the way the animal appropriates this food for nourishing its 

 body and the processes necessary to prepare the food for diges- 

 tion and assimilation. 



Digestion. — This may be defined as the physiological process 

 of preparing food or changing it into soluble substances that 

 may be absorbed, or taken into the circulation. 



Assimilation. — ^After the food is digested it is made use of 

 by the cells of the several tissues of the body. The acquiring 

 of the digested food for building up the several parts of the 

 body is called assimilation. 



Ferments. — In the processes of digestion foods are subjected 

 to changes which are destructive and beneficial. Before taking 

 up the several steps of digestion let us consider these changes 

 which are caused by ferments. 



A ferment is something which produces fermentation. When 

 it comes in contact with any feed stuff, new compounds are 

 formed and usually gas is given off. Examples of fermenta- 

 tion are; spoiling of butter, souring of milk, spoiling of fruits 

 in jars, spoiling of canned vegetables, and the converting of 

 apple juice to cider. 



Ferments not Beneficial to Digestion. — The above changes are 

 all due to the action of minute single celled organisms called 

 bacteria. These bacteria are present in very large numbers in 

 the alimentary canal and attack the food compounds, giving off 

 water, marsh gas, carbonic acid, ammonia, sulphuretted hydrogen 

 and other gases. In other words this kind of fermentation is 

 destructive and is always accompanied by a loss of food nutri- 

 ents. 



Ferments Beneficial to Digestion. — ^There is another class of 

 ferments which are chemical compounds and have the power 

 to change the composition of certain other substances simply by 



1 Adapted from Smith's Manual of Veterinary Physiology and from Dalrymple. 



