REGULATORS 603 



4. Life History. — The thymus reaches its greatest size, in 

 relationship to the weight of the body, about the time of 

 birth ; it continues to grow till puberty, when it begins to 

 atrophy, being replaced by fatty tissue. In adult life, it is 

 reduced to a mass of adipose tissue with only some islands of 

 thymus substance. Conditions of malnutrition lead to a tem- 

 porary atrophy of the gland. 



5. Physiology. — (1) Removal. — In young guinea-pigs this 

 produces no marked symptoms. In young dogs very different 

 results have been recorded by different investigators ; but the 

 most recent series of experiments shows no observable differ- 

 ence between normal pups and those deprived of their thymus. 

 Some investigators describe a peculiar sluggish condition with 

 manifestations of muscular fatigue ; others state that a condi- 

 tion of decreased calcification of the bones and the peculiar 

 enlargement of their ends, characteristic of rickets, are produced ; 

 but rickets develops very readily in puppies, and just as readily 

 in those with, as in those without, the thymus. A thymusless 

 pup may escape rickets while other members of the litter may 

 develop it. 



(2) Feeding tadpoles with thymus leads to continued growth 

 and absence of development, while feeding with thyreoid leads 

 to more rapid development. 



(3) After castration of male animals, the thymus persists in 

 adult life. If thymus and testes are both removed, the growth 

 of the animal is delayed. It would thus seem as if the thymus 

 and testes co-operate in stimulating growth, and that, if one of 

 these structures is removed, a compensatory hypertrophy of the 

 other occurs. As the testes increase in size, the thymus begins 

 to atrophy and to play a less important part. Similar relations 

 with the ovaries have not been established. 



8. Parathyreoids. 



1. Development. — These are formed as epithelial outgrowths 

 from the dorsal aspect of the third and fourth branchial clefts 

 on each side, there being thus two on each side. 



2. Position. — The parathyreoids formed from the third 

 clefts, in most animals, lie close to the thyreoid lobes, but 

 outside of them. Those from the fourth clefts are generally 

 embedded in them. In man, both sets lie outside of the 



