DIGESTION. 49 



to the quality or quantity, as to the absence of variety in the aliment 

 supplied. 



Aliments have been variously classified ; Prout divides them into 

 the following groups. Aqueous, including water, either alone, or 

 holdino- important elements in suspension or solution. Saccharine^ 

 including sugars, starch, gums, and vinegar. Oily or oleaginous, 

 including the various fats, oil, and alcohol. Albuminous, including 

 all those substances which contain nitrogen — such as fibrine, gela- 

 tine, albumen, caseine, and vegetable gluten. All the materials 

 which make up this group are derived generally from the animal 

 kingdom, with the exception of the last, which is contained in great 

 abundance in wheat. Similar, if not identical, principles exist in 

 vegetables. 



In milk we find a natural combination of all the various sub- 

 stances which contain nitrogen, and this is the only instance in 

 which nature has provided a single article of food for the support of 

 the animal body in which such an union exists. 



Liebig divides aliments into two classes. Azotised, and non- 

 azotised. The azotised products are for the nutrition and reparation 

 of the animal tissues, hence he calls them ''plastic elements of 

 nutrition:' The non-azotised substances are designed chiefly, ac- 

 cording to him, to supply the materials for animal heat and respira- 

 tion, hence he calls them " elements of respiratio7i:' The uncon- 

 sumed material being deposited as fat. 



Dr. R. D. Thomson proposes to call the azotised, nutritive ele- 

 ments, and the non-nitrogenised, calorifacie7it. 



The ori^anic compounds enumerated would be of little service 

 without the admixture of certain inorganic substances, which also 

 form a part of the animal frame, and which are constantly being 

 voided by the excretions. These are chloride of sodium, which 

 enters into the composition of the gastric juice, and the bile ; phos- 

 phorus, which is found in the osseous and nervous tissues ; sul^Jhur, 

 found in the albuminous tissues ; lime, which is required for the 

 consolidation of bone ; and lastly, iron, which will hereafter be 

 shown to be an essential constituent of heematosine. 



The digestive apparatus varies greatly with the habits of the 

 animal, and with the nature of the aliment with which it is supplied; 

 thus in carnivorous animals it is exceedingly simple, because the food 

 requires to undergo little change before it is fitted for nutrition. In 

 the ruminantia the food is macerated in a complex stomach, prior 

 to, as well as after it has been subjected to a more complete masti- 

 cation than is employed in other animals; because the aliment con- 

 tains but little nutritive matter, which is with difficulty separated 

 from it. In the omnivorous animal the digestive apparatus is mid- 

 way between those above described. 



Modifications also occur in the masticatory process ; the vegetable 



