630 INTERNAL SECRETION 



nuclein substances characteristic of the nuclear framework of the 

 true glands are much more abundant in the thymus than in lymph 

 glands. 



After a period of further development, which varies in duration 

 in different animals, the organ undergoes involution. In mammals 

 (including man) the thymus does not completely disappear in the 

 adult. Islands of thymus tissue are found at all ages among the 

 fat by which the bulk of the organ is replaced. It is usually stated 

 that in man the thymus begins to diminish in size about the end of 

 the second year, but the careful observations of Hammar indicate 

 that this is incorrect. According to him, the organ continues to 

 grow till puberty is reached, weighing on the average 13 grammes 

 at birth, 37 grammes at eleven to fifteen years, 25 grammes at 

 sixteen to twenty years, and only 6 grammes at sixty-six to seventy- 

 five years. Besides this involution with age, great changes in the 

 size of the thymus may occur at any time under the influence of 

 toxic substances or of deficient nutrition. In starvation, even in 

 the first three days of hunger, the weight of the thymus in rabbits 

 has been observed to shrink to one-half, and during prolonged 

 underfeeding even to one-thirtieth, of the normal (Jonson). The 

 opposite effect, namely, cessation of the involution process, or 

 even new formation of thymus tissue, may also occur, leading to 

 the presence of an unusually large so-called persistent thymus in 

 the adult. 



tThe point most clearly established in the physiology of the 

 ymus seems to be its relation to the sexual glandsA It is well 

 known that in castrated animals the thymus is larger and persists 

 longer than in entire animals. In bulls and unspayed heifers the 

 normal atrophy of the thymus, which begins after the period of 

 puberty, is greatly accelerated when the bulls have been used for 

 breeding, and when the heifers have been pregnant for several 

 months. There is a reciprocal influence of the thymus on the 

 testicles, and removal of the thymus before the time at which it 

 naturally atrophies is followed by a more rapid growth of the testes 

 (in guinea-pigs) (Pat on). IThe relation of the thymus to the growth 

 of bones is less well established, but according to some observers 

 extirpation of the gland retards their calcification. 1 

 f In young mammals the loss of the thymus is said to cause transient 

 disturbances of nutrition, a temporary decrease in the number of 

 all varieties of leucocytes, and a diminished resistance to the pus- 

 forming micrococcil probably connected with the relatively feeble 

 leucocytosis (or increase in the number of leucocytes) by which the 

 animals react to the infection. In the frog the thymus persists 

 throughout life. Yet the removal of it is not fatal if precautions 

 aarainst infection be taken. 

 IThe chief effect of intravenous injection of extract of human 



