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SCIENCE 



[N. S. Vol. XXV. No. 639 



size of the margins of safety. Structures, 

 for instance, which are to be employed for 

 alternating loads require high factors of 

 safety; the highest margin of safety is re- 

 quired when the structures are subjected 

 to rhythmic shocks. In constructing a 

 bridge or a machine it is then calculated 

 that the structures should be capable of 

 withstanding not only the stresses of rea- 

 sonably expected maximum loads, but also 

 the stresses of six or ten times the size of 

 such loads. The factor of safety has its 

 foundation in our ignorance of what might 

 happen and in the reasonable desire to meet 

 unexpected contingencies. Some writers 

 are therefore inclined to designate the fac- 

 tors of safety as factors of ignorance. 



It is obvious that the factors of safety 

 are applicable not only to the structures, 

 but also to the supply and expenditure of 

 energy of the machine. The supply of 

 fuel is calculated to have the engine in 

 readiness not only for expected maximum 

 work, but also to be capable of meeting 

 unexpected contingencies. On the other 

 hand, when there is no exceptional need 

 for it, no engine is allowed to perform 

 maximum work ; this economy here is again 

 a factor of safety. 



Are the structures and the functions of 

 the living animal body provided with such 

 factors of safety ? As far as I know, that 

 question has never yet been clearly raised, 

 and certainly was never made the subject 

 of a direct investigation. There is, how- 

 ever, no lack of casual remarks bearing on 

 that problem and these are manifestly un- 

 favorable to an assumption of the existence 

 or requirement of factors of safety in ani- 

 mal organisms. On the contrary, there are 

 many to whom it is apparently self-evident 

 that nature is economical and WEistes 

 neither material nor energy. Theories and 

 practical suggestions are based on such a 

 view as a premise which seems to their 



authors to require no special proof. Ver- 

 worn, for instance, asserts that the assump- 

 tion of special inhibitory nerves for skeletal 

 muscles can be rejected a priori, because 

 the presence of such nerves would be a 

 waste of matter and energy and in contra- 

 diction with the prevailing principle of 

 economy in the animal body. Another in- 

 stance is the extreme position held by some 

 recent writers with respect to the supply 

 of energy to the animal machine. Factors 

 of safety, maximum or optimum supply of 

 fuel, do not come in for a consideration in 

 the discussion of these writers. The argu- 

 ment is directed against the use of a dietary 

 standard which represents the average mean 

 supply of energy, the minimum supply 

 of food being considered as the ideal stand- 

 ard of diet. As is known to all of you, 

 Professor Chittenden and his co-laborers 

 have carried out nutrition experiments of 

 long duration upon a number of men. The 

 essential feature of these experiments was 

 the use of a low proteid diet; in some in- 

 stances the diet was also combined with a 

 considerable reduction in the calorie values 

 of the food. All the subjects of the ex- 

 periments retained their usual health. 

 Professor Chittenden admits that the diet 

 used in these experiments, especially with 

 regard to the proteid intake, represents the 

 minimum requirement of the human body; 

 he, nevertheless, earnestly advocates its 

 acceptance as a general standard of diet, 

 assuming a priori that the minimum food 

 with which a number of men can manage 

 to live for some time without harm is the 

 desirable standard of supply of energy for 

 all animal machines. Whereas in the econ- 

 omy of the human-made mechanisms and, 

 in fact, in the economies of all human 

 organizations, decrease in supplies and in- 

 crease in expenditure lead invariably to 

 disaster, it would seem that in the physio- 

 logical economy of the living mechanism 



