NATURE AND DESTINATION OF FOOD. 483 



by an excess in the amount of the fluid excretions ; and it may be satisfied 

 without the introduction of water into the stomach* ( 677). Thirst may also 

 be produced, however, by the impression made by peculiar kinds of food or 

 drink upon the walls of the alimentary canal ; thus salted or highly-spiced 

 meat, fermented liquors when too little diluted, and other similarly irritating 

 agents, excite thirst ; the purpose of which is obviously to cause ingestion of 

 fluid, by which they may be diluted. 



2. Nature and Destination of the Food of Jlnimals. 



640. The substances which are required by Animals for the development 

 and maintenance of their fabric, are of two kinds ; the Organic and the In- 

 organic. The former alone are commonly reckoned as aliments; but the lat- 

 ter are really not less requisite for the sustenance of the body, which speedily 

 disintegrates, if the attempt be made to support it upon any organic com- 

 pounds in a state of purity. In all ordinary articles of diet, however, the in- 

 organic matters are present in the requisite proportion ; and hence they have 

 very commonly escaped notice. The nature of these substances, and the 

 mode in which they are introduced into the body, will be considered here- 

 after ( 648). The Organic matters, used as food by Animals, are partly 

 derived from the Animal, and partly from the Vegetable kingdom ; and 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 ; consisting 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 che- 

 mical processes of a simple kind. 2. The Oleaginous group, including oily 

 matters, whether derived from the Vegetable kingdom, or from the fatty por- 

 tions of Animal bodies. The characteristic of this class, is the great pre- 

 dominance 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 Animal 

 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 Gelatine in their composition. These also 

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

 preceding. 



641. The compounds of the Saccharine group cannot, without undergoing 

 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 con- 

 vertible, within the Animal body, into those of the Oleaginous group; and, 

 like them, may be deposited in the form of Adipose matter. There is no 

 other tissue in the body, into which they can enter without considerable change ; 

 for all others are azotized; and it seems extremely improbable that non-azo- 

 tized compounds can, under any circumstances, be converted within the body 

 into compounds of the albuminous or gelatinous groups. 



This was among die remarkable results of the injection of fluid into the veins, in the 

 Asiatic Cholera. 



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

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

 most convenient hitherto proposed.. 



