156 THE VITAMINES 



applicable to all living organisms. As little as the modern clinician 

 can afford to ignore the physiological significance of the proteins or 

 of certain amino acids, just so little need he doubt the importance of 

 the vitamines for human nutrition. It is true that the conditions 

 leading to deficiency diseases in man are perhaps not so clear cut as 

 in animal experiments, since in the latter, we may choose such con- 

 ditions as will permit of the study of the significance of a single 

 constituent exactly. In spite of this, we do not yet know all there 

 is to be known of the laws of nutrition. It has been frequently 

 necessary to proceed empirically; for example, we fed white rice to 

 chickens and pigeons without knowing at the time tjiat this food- 

 stuff lacks other important constituents besides vitamine B. We 

 were still in the empirical stage when we wrote our first edition, but 

 since then, vitamine research has made great strides; nevertheless, 

 many of the findings obtained at that time, with few exceptions, 

 still endure to this day. Should it appear later that the human 

 avitaminoses are associated not only with a deficiency of the corre- 

 sponding vitamines, but with other correlated factors, then the 

 significance of vitamines for life would still retain its complete 

 justification. That the etiology in human avitaminoses may be more 

 complicated than appears at first glance, we have already pointed 

 out. Should it happen that there is a deficiency of one vitamine, 

 we may be almost certain that it is not the only fault in the diet 

 leading to the particular disease. Along with the lack of one vita- 

 mine, there may be a sub-optimal quantity of other vitamines; there 

 may also be a protein and salt deficiency. In addition to this, it is 

 possible that the quantitative relationship of the individual con- 

 stituents may not be correct; it may likewise be (and this is very often 

 the case in practice) that the vitamine is indeed present in the diet 

 but not in quite sufficient quantities. Related as it is to all these 

 factors, we must not wonder that the picture of an avitaminosis often 

 seems to us to be clouded. Taking rickets as a concrete example, 

 we see that although this disease is not of rare occurrence among 

 well-to-do people, it nevertheless occurs most frequently in the poorer 

 stratas of society, where poor living conditions and hereditary factors, 

 together with dietary causes, may play a part. It is also not likely 

 that the lack of vitamine A only is the cause of rickets, as is now 

 thought to be the case; it may be associated with a partial 

 deficiency of other factors, which chiefly consist in that the balance 



