794 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 



hyperplastic condition becomes fully developed, scarcely a trace of 

 iodine is contained in the gland. Later, when the hyperplasia gives 

 place to colloid goiter, the iodine increases again, both absolutely and 

 relatively. Moreover, it has been found that if iodide is administered 

 to an animal suffering from hyperplasia, the hyperplastic condition very 

 quickly disappears and the animal becomes normal. Thus, in brook 

 trout, the poor nutritive condition of the fish when hyperplasia has 

 developed can be immediately remedied by placing them in larger quan- 

 tities of running water or by adding small traces of iodide to the water. 

 The administration of small amounts of iodine as in ordinary salt from 

 salt deposits also prevents goiter in farm stock, this having been first 

 noted in the State of Michigan, where prior to the discovery of salt 

 deposits sheep breeding was an entire failure. The importance of admin- 

 istering small doses of iodides to school children living in goitrous dis- 

 tricts has recently been emphasized by Marine and Kimball. 39 As small 

 a dose as 0.001 gin. at weekly intervals prevents goiter in puppies sus- 

 ceptible to it. 



Feeding experiments carried out by Gudernatsch 40 and subsequently by 

 Kogoff and Marine, 41 indicate that the thyroid hormone has a powerful 

 influence upon the development of the body. Tadpoles fed upon thyroid 

 substance showed a striking acceleration of the normal metamorphosis. 

 Those fed with the glandular material grew less rapidly than the con- 

 trols fed upon ordinary diet, but the tails of the former showed more 

 rapid involution and the arm buds developed prematurely. 



Experimental Thyroidectomy 



A correct interpretation of the functional changes and symptoms which 

 follow upon partial or complete removal of the thyroid gland, or from 

 its disease, has proved a very difficult problem, partly because sufficient 

 care has not been taken to note how much parathyroid tissue was re- 

 moved along with the thyroid, and partly because the fact has been over- 

 looked that the effects produced by thyroidectomy and parathyroid- 

 ectomy are often very different in animals of the same kind at dif- 

 ferent ages. Speaking generally, it may be said that the influence of the 

 parathyroid is focused mainly on the nerve centers and only to a second- 

 ary degree on the metabolic functions, whereas the reverse is the case 

 with the thyroid, its main effect being on metabolism, although it prob- 

 ably also exercises a secondary effect on the nerve centers. More so 

 than in the case of any other endocrine organ, our knowledge concerning 

 the function of the thyroid has been gained by clinical experience, and 

 it is difficult to say whether the clinical or the experimental method has 

 contributed the greater amount of information. 



