COMPOSITION OF FOODS. 



Zl> 



The precise figure has caused some discussion. 

 At one time it was customary to multiply the 

 fats by 244, which is the greater proportion of 

 oxygen required for their combustion. Bauer 

 showed that this was too great, and believed that 

 in the animal body itself about 175 was the true 

 value. The best authorities now consider that 

 the correct figure is probably the equivalent of 

 heat produced by the two groups. According to 

 this, we must multiply by 225, or 2\, and we 

 may then add them to the carbo-hydrates, and 

 shall get the true " nutritive ratio." 



Some authorities — -chiefly chemists — intro- 

 duced another complication into the calculation, 



under the title of digestibility. 

 Digestibility. To quote a well-known writer*: — 



" The chemist first determines by 

 analysis the percentage of each of the nutrients 

 contained in the food. Weighed quantities of 

 the feed are then given to some animal, and the 

 solid excrement voided during the trial is saved, 

 weighed, and samples of it analysed. Knowing 

 how much of each nutrient was fed and how 

 much of it reappears in the solid excrement, 

 tlie difference is held to be the portion digested, 

 since it must have been retained in the body." 

 Many American poultry dietaries have been 

 calculated upon this principle; the real analysis 

 being revised by laborious calculation, and the 

 figures reduced by what is termed a " digestive 

 coefficient " obtained in this manner. But the 

 whole is a mistake, based on ignorance of 

 physiology. Its only basis is the fact that in 

 the case of animals whose food largely consists 

 of fibre and hard cellulose, by some of them 

 scarcely any of this is digested, and it appears in 

 the excrement in visible form, of which horse 

 manure is a familiar example. Ruminants, 

 which subject the fibre to long softening before 

 rumination, digest a considerable portion even 

 of such materials, and so do birds, which soften 

 it in the crop and grind it in the gizzard. But 

 in such a case as that of man, who rejects such 

 material from his food before eating, the amount 

 of solid excreta has absolutely no relation 'what- 

 ever to indigestibility. Any medical man knows 

 of cases in which no evacuation may have taken 

 place for a fortnight or more, though there has 

 been fair activity, and a quite ordinary amount 

 of food has been consumed ; whilst in an ordin- 

 ary case many pounds weight would have been 

 excreted in the same time. The last case, and 

 not the first, will be that of the best digestion ; 

 and the solid excreta, equally with the liquid, 

 are in their nature not qiaterial which could not 

 be digested, but secretions through which the 



* W. A. Henry, " Feeds and Feeding." 



body excludes its used-up products : the pro- 

 ducts of its vital processes, and of food which 

 has been effectually digested, and done work in 

 the manifold changes through which it has 

 passed while within the system. We shall keep 

 nearest to the truth so far as known at present, 

 as well as simplify our work, by classing the 

 crude fibre or husk by itself as more or less 

 indigestible, and basing our dietary upon the 

 rest, letting any nutrient there may be in the 

 husk go in addition. It is also to be remembered 

 that this component of poultry food is almost 

 always more or less laxative in tendency. 



On this basis, then, we deal with foods. The 

 following table gives the principal materials 

 available for poultry-feeding, roughly classified, 

 and showing their composition as above de- 

 scribed ; and the amount of fats and oils is 

 further shown as multiplied by 2J, in order that 

 this product may be used for calculating the 

 nutritive ratio. 



