INTRODUCTION 33 



present is at once distributed throughout the body and the maximal 

 effects are seen before it is destroyed or excreted. A large proportion of 

 our significant data as regards suprarenal physiology have been derived 

 from the study of the results of intravenous injection. 



Administration by Mouth. The administration of gland extracts by 

 mouth has been of great use clinically, as in the thyroid treatment of 

 myxedema and cretinism. Various other examples of the effectiveness 

 of oral administration are discussed in the chapter on Opotherapy. In 

 experimental work the method has been most useful in the study of the 

 hormone factors in growth. Guddernatsch's well-known experiments on 

 the influence of thyroid substance on the metamorphosis of tadpoles is a 

 case in point. Numerous other examples will be found in the chapters 

 on the physiology of the various endocrin glands. In such work the 

 possible importance of nonspecific substances in the material fed should 

 always be kept in mind. The presence of nucleoproteins may be an im- 

 portant factor in results. If the experimental animal were not receiving 

 otherwise an adequate supply of protein, the administration as gland 

 substance of relatively small quantities of this important food element 

 might cause considerable change in growth. The researches of Funk, 

 Mendel, McCollum and others show that mere traces of certain as yet 

 unknown substances may exercise a profound influence upon the growth 

 and developmental processes. These substances, the vitamins, as Funk 

 calls them, must always be excluded as the determining factor in results. 

 In all feeding experiments the control animals should be supplied with 

 some indifferent tissue substance equivalent to the gland material admin- 

 istered to the experimental animals, or else both experimental and control 

 animals should receive a maintenance ration so complete that the addition 

 of the gland substance can play no significant role merely as food or as a 

 conveyor of vitamins. Another source of error is growth experiments 

 resides, according to Robertson (1917), not in the substance fed but in a 

 failure to recognize certain essential features in the process of growth 

 itself. a There is," according to this investigator, "a widespread and mis- 

 taken tendency to regard growth as a single process and to infer that, if a 

 given substance or condition accelerates the growth of one particular 

 tissue at any given time, the same substance or condition will also accel- 

 erate the growth of other tissues or of the same tissue at a different 

 physiological age. Growth, on the contrary, is a multiplex phenomenon 

 and factors which favor the growth of one particular type of tissue or the 

 growth of the whole animal at one particular age may actually exert the 

 reverse action upon another tissue or at another stage in the growth of 

 the animal concerned." 



The necessity of controlling rigidly the environmental factors would 

 seem so obvious as to require no mention. Notwithstanding, there can be 

 found in the literature more than one elaborate report of researches in 



