xii INTERNAL SECRETION AND DUCTLESS GLANDS 223 



thyrin, thyroiodin). Little is known of its function in the 

 frog. In the higher vertebrates its removal is followed in 

 nearly all cases by fatal effects. Removal of only a part 

 of the gland, as a rule, creates but little disturbance. Life 

 may be maintained for a considerable period after complete 

 removal of the thyroid, by giving injections of extracts of the 

 gland into the blood. In man 

 the disease called myxcedema or 

 cretinism, caused by the atrophy 



of the thyroid, is often much S* Vfc — J **~ y W^*Th. 

 helped or even cured by the 

 administration of thyroid extract. 

 The substance to which the thy- 

 roid owes its important function is , 



r FIG. 62. — Diagram showing the 



a proteid with which a compara- position of the thymus," ih. 

 tively large amount of iodin is in D » l < depressor mandibuiae 



, . rr, ir 1 .1 . muscle; Ty, tympanum. 



combination. I reupel found that 



frogs from which both thyroids were removed lived only two 

 or three days, but he was not entirely certain that the result 

 might not be due to effects of the operation other than the 

 loss of the parts in question. 



The Thymus. — The thymus is a small, oval organ, some- 

 what reddish in color, situated behind the tympanic mem- 

 brane under the depressor mandibuiae muscle. As in most 

 higher forms, the thymus diminishes in size with age. Maurer 

 found that in Rana esculenta the thymus attained its maxi- 

 mum size in specimens of two or three centimeters in length. 

 In old frogs (7 to 8 cm.) the organ is much smaller and shows 

 marks of degeneration in structure. 



The thymus has essentially the structure of a lymphoid 

 gland. In its fine network of adenoid tissue lie numerous 

 small, rounded cells. There are also several large cells of 

 concentric structure concerning whose origin and significance 



