ON DIET. 513 



some special connection between visual sensations and the nutrition of the 

 skin ; and this can hardly be other than a nervous connection. The effects 

 of prolonged darkness on nutrition in general and the experimental results 

 which show that the total metabolism of the body is influenced by light, also 

 suggest some nervous action. The influence of cold, again, in determining 

 the growth of hair points in the same direction. 



Making every allowance for the intervention in the production of the 

 phenomena quoted above of such factors as common actions of the nervous 

 system already well known to us, such as vasomotor changes, making every 

 allowance for the consequences of the failure or bluntness of sensation and 

 the absence of those beneficial after-results of muscular activity which we 

 pointed out in 86, recognizing, moreover, that changes in one organ may 

 affect the condition of other distant organs by changes induced in the com- 

 position or qualities of the blood, there still remains a residue which seems 

 distinctly to point to the conclusion that the influence of the nervous system 

 is not limited to such changes of the muscles as belong to the production of 

 contractions or the generation of heat, but bears on the whole nutrition of 

 the muscle. Similar considerations lead us also to conclude that the influ- 

 ence of the nervous system bears on the whole nutrition of the glands, of the 

 bloodvessels, of the skin, and of the connective tissue in general, in fact of 

 nearly the whole body. 



ON DIET. 



463. An ordinary man living an ordinary life will need for the main- 

 tenance of vigorous health a certain amount of food of a certain kind ; this 

 we may take as a normal diet. 



Presuming that the experience of man has led him to adopt what is good 

 for him, we may ascertain approximately the normal diet by means of the 

 statistical method, by examining the nature and amount of the daily food of 

 a very large number of individuals. The most valuable data for this pur- 

 pose are those gained by inquiries among persons who choose their own food ; 

 the results gained from the diets used in prisons or other institutions, or 

 among bodies of men such as the army, though more readily arrived at, are 

 open to the objection that the diets in question are determined in part by the 

 theoretical opinions of those whose duty it is to fix the diet. Putting together 

 the various statistical results thus obtained, and selecting the quantities which 

 seem to be most commonly used rather than attempting to strike a strict 

 average or take a strict mean, we find that in an ordinary diet for the twenty- 

 four hours the several food-stuffs are : 



Proteids , from 100 to 130 grammes. 



Fats " 40 to 80 



Carbohydrates " 450 to 550 



to these we must add 



Salts 30 grammes. 



Water 2800 " 



The total (available) potential energy of the lower estimate is 2610, of the 

 higher 3505 (kilogramme-degree) calories, calculated, in round numbers, on 

 the data of 441. With such a statistical diet we may compare an experi- 

 mental diet, that is to say a diet arrived at through a series of trials on an 

 individual man whose body might be taken to be an average one, that diet 

 being considered a normal one in which the body, maintaining vigorous 



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