INTERNAL SECRETIONS 89 



activity of other organs. Formerly the several parts of the 

 body were looked upon as independent. Their activity was 

 regarded as a direct response to the commands of the nervous 

 system. If it varied in kind, the variations were supposed 

 to depend upon the quality of the nervous impulses which 

 reached the organ. Evidence is rapidly accumulating that 

 many exhibitions of function are evoked by the calls of " hor- 

 mones," or chemical messengers, not by command of the 

 nerves. 



Internal secretions, using the expression in its general 

 sense, are necessary for the co-ordination of the work of the 

 various parts of the animal mechanism. Colloid is the internal 

 secretion of the thyroid gland and of the parathyroids. Unlike 

 most other internal secretions, it is a substance easily analysed, 

 and startlingly definite in its chemical characteristics. It is 

 composed mainly of a protein which contains iodine. From 

 this protein a substance termed " iodothyrin " may be ob- 

 tained, of which no less than 10 per cent, is iodine ; but it is 

 uncertain whether iodothyrin is preformed in the gland. The 

 exact nature of the active substance of the internal secretion 

 of the thyroid gland matters little. Whether it be iodothyrin 

 or a protein, its activity depends upon the fact that it contains 

 iodine in large quantity. Iodine amounts to from 0-3 per 

 cent, to 0-9 per cent, of the weight of the whole thyroid gland 

 in Man. 



Iodine is very widely distributed in Nature. It is present 

 in the air, in rain-water, in herbage. It is also present in all 

 parts of the body, although in quantities which are infinitely 

 minute. It is found in sea-water, and is relatively abundant 

 in marine plants. There is no reason for supposing that it 

 is deficient in districts in which goitre is common. It would 

 appear more likely that the soil has properties which result 

 in the fixing of iodine in a form in which it is not available 

 for plant-food, and that in consequence animals are unable 

 to obtain a sufficient supply. Careful analyses have shown 

 that the thyroid glands of sheep bred in mountainous districts 

 where goitre is common contain but one-thirtieth part as 

 much iodine as the thyroid glands of sheep bred in places where 

 goitre is rare. In ancient times burnt sponge and seaweed 

 were esteemed useful in the treatment of goitre. Later, iodide 



