320 OF DIGESTION AND NUTRITIVE ABSORPTION. 



CHAPTER VIII. 



OF DIGESTION AND NUTRITIVE ABSORPTION. 



I. Nature and Destination of the Food of Animals. 



430. Before we examine the nature of the process by which the food of 

 Animals is prepared for absorption into -their bodies, it will be desirable to con- 

 sider the characters of the aliment itself, and the purposes to which it is to be 

 applied. Strictly speaking, the term aliment may be applied to all those sub- 

 stances, which, when introduced into the living body, contribute to its growth, 

 or to the repair of the losses which it is continually sustaining.* But in gene- 

 ral the term is used in a less extensive sense ; being confined to those mate- 

 rials which are absorbed and applied to the purpose of the economy, only after 

 undergoing the process of Digestion; and in this sense it will be here employed. 

 It has been mentioned ( 14) as a general principle, that, whilst Plants have 

 the power of combining the elements supplied by the inorganic world, into 

 the materials of their nutrition, Animals can only subsist upon substances 

 that have been already so combined, and that have previously formed a part of 

 some organized body, either Vegetable or Animal. All the materials used as 

 food, therefore, by Animals, are derived from one or other of these two king- 

 doms. They may be conveniently arranged under the four following heads.t 

 1. The Saccharine group, including all those substances derived from the 

 Vegetable kingdom, which are analogous in their composition to Sugar ; con- 

 sisting of oxygen, hydrogen, and carbon, alone ; and having the two first 

 present in the proportions to form water. To this group belong starch, gum, 

 woody fibre, and the various tissues of Plants ; which closely resemble each 

 other in the proportion of their elements, and which may be converted into 

 Sugar by chemical processes of a simple kind. 2. The Oleaginous group, 

 including oily matters, whether derived from the Vegetable kingdom, or from 

 the fatty portions of Animal bodies. The characteristic of this class, is the 

 great predominance of hydrogen and carbon, the small proportion of oxygen, 

 and the entire absence of nitrogen. 3. The Albuminous group, comprising 

 all those substances, whether derived from the Animal or Vegetable kingdom, 

 which are closely allied to Albumen, and therefore to the majority of the Ani- 

 mal tissues, in their chemical composition. In this group, a large proportion 

 of azote is united with the oxygen, hydrogen, and carbon of the preceding. 

 4. The Gelatinous group, consisting of substances derived from Animal bodies 

 only, which are closely allied to Gelatin in their composition. " These also 

 contain azote ; but the proportion of their components differs from that of the 

 preceding. The compounds of the Saccharine group cannot, without under- 

 going a metamorphosis, form part of any Animal tissue ; as there is none 

 which they resemble in composition. It will be shown, however, that they 

 are convertible, within the Animal body, into those of the Oleaginous group ; 



* Thus, Water and Air may be termed aliments. 



f Dr. Prout's classification of alimentary substances is here adopted, with a slight 

 modification; not as being altogether unexceptionable, but as being, in the Author's 

 opinion, the most convenient hitherto proposed. 



