418 MANUAL OF PHYSIOLOGY. 



3. We have just seen that the relative amounts of outgoings 

 and of material laid by as store are altered and regulated by the 

 quantity of income. And we find that the quality of the income, 

 i.e., the relative proportions of the various food-stuffs, has a ma- 

 terial influence on the quantities of material laid by and elimi- 

 nated respectively. We must, therefore, consider the efficacy of 

 each of the groups of the food-stuffs when employed alone and 

 mixed in different proportions. 



4. Different animals seem to have different powers of assimila- 

 tion ; and under various circumstances the requirements and 

 assimilative, power of the same animal may vary. 



An animal fed upon a purely meat diet requires a great amount 

 of it to sustain its body-weight. It has been found that from .^ 

 to 2 ! 7 of the body-weight in lean meat daily is necessary to keep, 

 an animal alive without either losing or gaining weight. If more 

 than this amount be supplied the animal increases in weight, and 

 as its weight increases a greater amount of meat is required to 

 keep it up to the new standard. So that to produce a progressive 

 increase of weight with a purely meat diet, it is necessary to keep 

 on increasing the quantity of meat given. The reason of this is 

 found in the fact that albuminous diet causes an increase in the 

 changes occurring in the nitrogenous tissues. 



If an animal which is in extremely poor condition be given an 

 ad libitum supply of lean meat, only a limited portion of the albu- 

 minous substance is retained in the tissues. By far the larger 

 proportion of the nitrogenous food is found given off and is repre- 

 sented in the urine by urea, and a comparatively small proportion 

 is stored up. If this large supply of meat diet be continued for 

 some time, less and less of the albuminous material is stored, more 

 and more being eliminated as urea, until finally the urea excreted 

 justs corresponds to the albuminous materials in the ingesta. 

 When only meat is given, it must be supplied in large quantities 

 to maintain the balance of the nitrogenous income and expenditure 

 which is spoken of as nitrogenous equilibrium. Upon the occur- 

 rence of a change in the amount of nitrogenous'Jngesta, this nitro- 

 genous equilibrium varies, and it takes some time to become rees- 

 tablished, because a decrease in the meat diet is accompanied by 



