492 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



Casein is an inadequate protein on account of its deficient content of 

 cystine (Fig. 34). In milk this deficiency is supplied by Lactalbumin. 

 In addition to the various dietary constituents which have been 

 definitely ascertained to be essential and irreplaceable there are others 

 which we can infer, from known data, to be equally essential. Thus 

 Cholesterol has been shown by Gardner and his collaborators not to be 

 synthesized by animal tissues; the cholesterol in the blood and tissues 

 being proportionate to the cholesterol derivable from the dietary. 

 Now cholesterol is an essential constituent of nervous tissues, and 

 derivatives of cholesterol, such as the bile-acids, play an essential part 

 in the bodily economy. A diet lacking in cholesterol, which is not at 

 the same time lacking in other essential dietary constituents, is very 



-soo 



Mo 

 Uo 



IfO 



llo 

 too 

 no 



11,0 



IVO 



no 



/CO 



to 



to 

 to 



Each division - to day* 



Days 



Each division - 20 days 



FIG. 34. Curves of growth of rats on basal rations plus casein, showing effect of 

 addition of cystiiie to an inadequate allowance of casein. (After Osborne and Mendel.) 



difficult to devise, so that direct proof of its essentiality in the dietary 

 has not yet been adduced. Still we may infer with a fair degree of 

 confidence that a certain minimal content of cholesterol in the diet is 

 requisite, if not for maintenance, then at least for normal growth and 

 development. 



From these various investigations it is clear that the elementary 

 substrates of growth are in all probability very numerous and represent 

 a variety of chemical genera, or at any rate, lipoidal substances, basic 

 substances and amino-acids, superadded to the more elementary 

 requirements of nitrogen, carbon and calories. 



It is furthermore evident that growth, like all other chemical trans- 

 formations, is absolutely dependent upon its raw materials or sub- 

 strates, and cannot occur in their absence. On the other hand, the 



