METABOLISM 



589 



together with fresh vegetables such as carrots once or twice a week. 

 Nursing mothers and young rats receive also a paste containing milk 

 powder 60 parts, starch 12 parts, and lard 28 parts. Experimental 

 diets, whenever possible, are made into a semi-solid paste, which 

 the rats cannot easily scatter. Dry food mixtures may be kept for 

 some weeks. 



Animals are most conveniently weighed on a spring balance reading 

 from o-icoo gm. in 5 gm. divisions, weights being estimated to the 

 gram. Food cups with food are most readily weighed on a special 

 spring balance. 1 Animals from 50-80 gm. in weight are generally 



FIG. 178. GUINEA PIG WITH SCURVY. SHOWING "FACE ACHE POSITION." 

 (Special Report British Medical Research Committee, No. 38, 1919.) 



used for feeding experiments. They may be marked by clipping the 

 ears with a small shears or by staining patches of the fur, and are 

 weighed once a week. 



2. Influence of Protein (Amino Acid) Deficiency. At least four of 

 the essential amino acids which occur in protein substances cannot 

 be synthesized in the animal body. These are cystine, lysine, trypto- 

 phane and tyrosine. The acids mentioned must, therefore, be included 

 in our diet if we are to be properly nourished. The following experi- 

 ments, which may readily be made using white rats as subjects, will 

 clearly demonstrate the importance of two of these amino acids, i.e., 

 cystine and lysine. 



(a) Demonstration on Cystine Deficiency. Place two young white rats 

 (40-60 grams) in separate cages (see Fig.fiSo). Feed one rat Diet i and the 

 other rat Diet 2 as listed in the following table : 



x The rat balance may be obtained from the Chatillon Company, New York, and the 

 food balance, from Charles Forschner and Sons, New York. 



