246 OF FOOD AND THE DIGESTIVE PROCESS. 



place upon the quantity of solid matter they contain ; being of course 

 the greater as the solids form the larger proportion of the entire weight. 

 Many esculent vegetables contain so large a quantity of water, that the 

 nutriment they afford is very slight in proportion to their bulk. Next 

 it depends upon the proportion of digestible matter which the solid parts 

 include ; for it is not every substance containing the requisite ingre- 

 dients that is capable of being reduced to a state which enables it to be 

 absorbed. Thus woody fibre is composed of the same elements as 

 starch-gum ; but it passes out of the intestinal canal unchanged, and 

 therefore affords no nutriment. In the same manner, the horny tissues 

 of animals, though nearly allied to proteine in their composition, are 

 completely destitute of nutritive properties to man and the higher ani- 

 mals, because not capable of being reduced by their digestive process ; 

 though certain insects appear capable of living exclusively upon them. 



428, But when the watery and indigestible parts of the food are put 

 out of consideration, and our attention is directed only to the soluble 

 solids, we find a most important difference in the chemical composition 

 of different substances, which renders them more or less appropriate to 

 the different purposes which have to be answered in the nutrition of the 

 body. It has been already pointed out, that Vegetables possess the 

 power of combining the elements furnished by the inorganic world into 

 two classes of compounds, the ternary, consisting of oxygen, hydrogen, 

 and carbon, and the quaternary, which consist of these elements, with 

 the addition of azote or nitrogen. These two classes are hence termed 

 the non-azotized, and the azotized. 



429. Now the azotized compounds are required for the reparation of 

 the waste of the muscular tissue, and for the general nutrition of the 

 body; consequently, unless the food contain a sufficient proportion of 

 these substances the body must be insufficiently nourished, and the 

 strength must diminish, even though other elements of the food be in 

 superabundance. The azotized substances formed by Plants are essen- 

 tially the same, as already shown ( 174), with those which are fur- 

 nished by the Albuminous solids and fluids of Animals ; but the quantity 

 of them is usually small in proportion to the non-azotized, being consi- 

 derable only in the Corn-grains, in the seeds of Leguminous plants, and 

 in some other products, which the universal experience of ages has 

 demonstrated to be the most nutritious of Vegetable substances. The 

 other azotized compounds existing in the animal body may be elaborated 

 by the transformation of these proteine-compounds ; so that when they 

 are duly supplied, the system cannot become enfeebled for want of sup- 

 port. But there is another azotized compound, (jelatine, that is fur- 

 nished by Animals, to which nothing analogous exists in Plants ; and 

 this, although it cannot sustain life by itself, is a valuable adjunct to the 

 proteine-compounds. For as the gelatinous tissues suffer waste, in com- 

 mon with the others, it is evident that if the gelatine be supplied already 

 prepared, it may be at once applied to their nutrition ; and thus the pro- 

 portion of proteine, which they would otherwise require, is not demanded, 

 and the labour of transformation is also saved. Further, there is this 

 great ^ advantage in combining a proportion of gelatine with the food, 

 especially when the digestive powers are feeble, that being already in a 





