FOOD REQUIREMENTS. 421 



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 

 material influence on the quantities of material laid by and 

 eliminated 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 as- 

 similative 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 7^ to ^V f tne 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 

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

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

 resented in the urine by urea, and a comparatively small propor- 

 tion 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 just corresponds to the albuminous materials in the in- 

 gesta. 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 occurrence of a change in the amount of nitrogenous ingesta, 

 this nitrogenous equilibrium varies, and it takes some time to 

 become reestablished, because a decrease in the meat diet is 



