CHAPTER XVIII 



EFFECTS ON THE THYMUS 



The thymus is an organ characteristic of the growth period, and is of especial 

 interest in pediatrics on account of its pronounced tendency to atrophy in all 

 conditions of inanition or malnutrition. Following a brief summary, the effects 

 of inanition upon the thymus will be considered in detail under (A) total inani- 

 tion, and (B) partial inanition. 



Summary of the Effects on the Thymus 



The weight of the thymus responds so promptly and extensively to conditions 

 of malnutrition as to justify fully Simon's designation of it as a "barometer of 

 nutrition." In this respect, it is rivalled only by the adipose tissue. The 

 loss is relatively far greater than that in the whole body. In both man and 

 lower animals the loss in thymus weight usually reaches at least 75 per cent 

 before death from acute inanition (total complete, or on water only) ; and in 

 chronic forms (incomplete total) may reach 90 to nearly ico per cent, though it 

 is doubtful whether it ever completely disappears as sometimes stated. 



The thymic atrophy during inanition occurs at every age, but is especially 

 notable is the young, where the thymus is both relatively and absolutely larger 

 than at later ages. The persistent growth tendency found during inanition in 

 many of the organs during infancy does not appear in the thymus, although its 

 loss in the newborn rat appears relatively less than at later ages. 



The characteristic thymic atrophy appears not only in the various degrees of 

 total inanition, but also in most forms of partial inanition, as well as in many 

 other conditions (exhaustion, disease, etc.), and is designated by Hammar as 

 "accidental involution," in contrast with the normal "age involution" of the 

 thymus. 



In most cases the degree of thymic atrophy appears closely correlated with 

 the general malnutrition of the body; although certain exceptions occur, in 

 which the thymus may be large in spite of general emaciation. This is found in 

 some cases of infantile atrophy, and has been claimed in human rickets, but is of 

 doubtful significance. 



Recovery of the normal thymic weight usually occurs promptly upon refeed- 

 ing, unless the inanition has been extremely severe. There is some evidence 

 indicating persistent subnormality in the thymus of rats permanently stunted 

 by underfeeding, but this is still somewhat uncertain. 



The histological changes in the thymus during inanition are equally striking 

 and characteristic. The resultant involution affects all parts of the thymus ; but 

 especially the lymphoid tissue, which here (as elsewhere throughout the body) 

 undergoes a pronounced atrophy. Although there are numerous variations in 



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