PHYSIOLOGY AND EXPERIMENTAL PATHOLOGY 377 



planations must be seriously considered in the interpretation of all posi- 

 tive experimental findings, and that for the interpretation of the positive 

 experimental findings reported by some investigators those explanations 

 become absolutely essential." On the whole, the available evidence from 

 extirpation experiments quite fails to prove that the thymus has any en- 

 docrin function. 



Results of Experimental Methods Other Than Extirpation 



Several observers have studied the effects of grafting thymus tissue 

 into other animals. These investigations have been directed particularly 

 to forestalling the assumed deleterious effects of thymectomy. If thy- 

 mectomy, however, has per se no deleterious effects, such researches of 

 course fall to the ground. At any rate, grafting experiments have failed to 

 contribute anything significant to our present-day knowledge of thymus 

 function. 



Relatively few investigators have studied the effects of thymus feed- 

 ing in mammals. E. R. Hoskins (1916) reported completely negative re- 

 sults in rats. Gudernatsch's experiments with tadpoles have been widely 

 quoted. Later investigations seem to indicate that Gudernatsch's results 

 were due merely to the food factor. For a fuller discussion of the results 

 in amphibia, the chapter on "Endocrin Factors in Growth and Develop- 

 ment' 7 may be consulted. 



Injection experiments also have failed to contribute any significant 

 data on thymus function. Thymus extracts, when injected into the blood- 

 stream, give merely such reactions as can be obtained from non-specific 

 tissue materials in general (Vincent and Sheen). 



Much stress has been laid on the early postpuberal involution of the 

 thymus, as indicating that this gland plays an important role in bringing 

 about the changes characteristic of puberty. As Hammar has pointed out, 

 however, involution occurs to a much less extent than is often supposed. 

 While it is true that the thymus weight in adults, as reported in the 

 literature, is relatively much less than in youths, this can be accounted 

 for to a considerable extent by the fact that the data had been derived from 

 autopsies following death from various wasting diseases. The thymus, 

 being especially liable to inanition atrophy, is in such cases much below 

 the normal adult weight. When the glands are collected from subjects 

 who have died suddenly in a state of good nutrition, they show to a very 

 considerable degree the same morphological structure as in youth. What- 

 ever function the thymus may have, therefore, probably persists well past 

 the puberal stage. 



