376 OF FOOD, AND THE DIGESTIVE PROCESS. 



rine 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 first two present in the proportions 

 to form water. To this group belong starch, gum, woody fibre, and the cellulose 

 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 ; 

 whilst Alcohol, which is derived from Sugar by the process of fermentation, has 

 a composition which rather connects it with the next group. 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 lies 

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

 oxygen, and the entire absence of nitrogen. 3. The Albuminous group, com- 

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

 kingdom which are closely allied to Albumen, and through it to the Animal 

 tissues generally ( 20), in their chemical composition. In this group, a large 

 proportion of azote is united with the oxygen, hydrogen, and carbon of the pre- 

 ceding. 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. There are many other substances, however, which, though truly 

 alimentary, and consumed to a considerable amount, cannot be legitimately 

 placed under either of the above heads } such are, for example, the Vegetable 

 Acids. 



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

 metamorphosis, form part of any Animal tissue ; as there is none which they at 

 all resemble in composition. It has been shown, however, that they are con- 

 vertible, within the animal body, into those of the Oleaginous group ( 40), and 

 may thus, like the latter, be applied to the formation of the Adipose and Ner- 

 vous tissues. But the amount of these substances which is thus employed, is a 

 very small part of that which is ordinarily introduced as food; and by far the 

 larger proportion of them is made subservient to the maintenance of the Heat 

 of the body by the combustive process. The Sugar which is taken in as such, 

 being dissolved and absorbed into the current of the circulation, appears to 

 undergo a speedy metamorphosis into lactic acid, which is the form under which 

 it is finally oxidized and burned off ( 49) ; and Starch is made capable of 

 undergoing the same change, by being first converted into Sugar during the 

 digestive process. Oleaginous matters do not seem to undergo any change pre- 

 liminary to their oxidation, save their reduction to a state of very fine division. 

 We shall presently see ( 401) that a very considerable difference exists 

 between the Saccharine and Oleaginous matters, in regard to their relative 

 calorifying powers. That none of these non-azotized substances can be made 

 capable, by metamorphosis or combination within the Animal body, of taking 

 the place of the azotized substances as " histogenetic" or " plastic" compounds, 

 may now be regarded as one of the most certain facts in Physiology; the con- 

 current evidence of experiment and observation leading to the conclusion that 

 in Plants alone can any production of azotized compounds take place, and that 

 Animals are in consequence directly or indirectly dependent upon the Vegetable 

 kingdom for their means of subsistence. If animals be fed exclusively upon 

 Saccharine or Oleaginous substances, of any kind or in any combination what- 

 ever, they speedily perish with symptoms of Inanition ; and the only assistance 

 which such food affords in the prolongation of life, is derived from its calorific 

 power ( 375). 



399. The application of the substances forming the Albuminous group, to 

 the support of the Animal body, by affording the materials for the nutrition 

 and re -formation of its tissues, and also for the maintenance of its heat, has 



