644 INTERNAL SECRETION ENDOCRINE GLANDS 



different from the cells of lymph glands. Chemical differences also 

 exist. For example, 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 (in- 

 cluding 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. In certain diseases, as exophthalmic 

 goitre, myxoedema and Addison's disease, an actual regeneration of 

 the involuted thymus may occur. 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 

 -the so-called status thymicus, or status lymphaticus. 



The point most clearly established in the physiology of the thymus 

 seems to be its relation to the gonads or sex glands. 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 acce- 

 lerated 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) (Paton). The remarkable 

 atrophy or involution of the gland at puberty is a striking indication 

 of this relation. That the nexus between the thymus and the gonads 

 is chemical and not nervous is shown by the fact that when the thymus 

 is autografted under the skin (a situation in which grafts readily take 

 and grow after removal of the main portion of the gland in sexually 

 immature rabbits) these grafts behave in the same way as the gland 

 in situ, showing earlier involution both in the male and in the female 

 when the animals are allowed to breed (Marine and Manley). The 

 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. 



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

 disturbances of nutrition, as temporary decrease in the number of 

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

 forming micrococci, probably connected with the relativelv feeble 

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

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

 out life. Yet the removal of it is not fatal if precautions against in- 

 fection be taken. The contention that the thymus is indispensable for 



