THE FROG 53 



Digestion. 



We must now turn our attention briefly to the actual process 

 of digestion itself, and in doing so we shall consider this process 

 in general, and not in the frog in particular. The chief point 

 of difference between the process in the frog and the rabbit is 

 that the former does not possess any salivary glands, and hence 

 the digestion does not actually commence in the mouth. In 

 correlation with this we find that the frog does not masticate its 

 food. Whether the animals considered be carnivorous, i.e. flesh- 

 eating, or herbivorous, i.e. plant-eating, the reactions are funda- 

 mentally the same, for although the actual food substances differ 

 slightly in the two cases they are closely allied chemically. The food 

 taken in consists of matter that is, or has been, living, and the 

 greater part of it falls into a small number of groups of substances, 

 sometimes spoken of as the proximate principles. One most im- 

 portant constituent of all food is water, in addition to which we 

 find: 



1. Mineral salts, such as common salt, etc. 



2. Proteins, i.e. compounds formed by living matter and contain- 

 ing Nitrogen. Ultimate analysis shows them to contain Carbon, 

 Hydrogen, Oxygen, Nitrogen and Sulphur in various proportions. 



3. Fats, i.e. compounds of fatty acids with Glycerine.* 



4. Carbohydrates, i.e. compounds of Carbon, Hydrogen and 

 Oxygen, in which the last two are in the same proportion as in 

 water, f 



The first group contains inorganic substances, and the last three 

 organic, that is to say, substances that in nature only occur in con- 

 nection with living matter. Nearly all of them are in solution, or 

 readily soluble. Matter in solution falls into two classes, according 

 as to whether it can or cannot pass through an organic membrane. 

 If a vessel be divided into two compartments by means of an organic 

 membrane (e.g. parchment) , and a solution of common salt be poured 

 into one side and ordinary water into the other, it will be found 

 after a time that salt is present in both compartments. It will 

 continue to pass through the membrane until the solution on both 

 sides is of equal strength. Such a passage is termed osmosis or 

 dialysis, and a compound capable of performing it is distinguished 

 as a crystalloid, as such substances are usually easily obtainable in a 

 crystalline condition. The other class of substances would be quite 



* The fatty acids are a series of acids derived from the oxidation of mono- 

 tomic alcohols. 



f This definition is convenient rather than accurate, for while including the 

 Carbohydrates it also includes a few compounds like acetic and laetic^acids 

 that are not carbohydrates. 



