THE THYliOll) <;LA.\I) 



257 



Histiocytes and lympliot-vtes occur in tiic intcrfoilicular connec- 

 tive tissues and also nerves from syni])athetic tjanfflia. Although 

 some investi<jators regard colloid as a waste material, others are 

 inclined to view it as a reserve secretion which passes out into the 

 vascular network to find its way into the general circulation. It 

 has been ascertained that the thyroid collects iodine from the 

 blood and forms an organic compound, calletl thyroxin, which regu- 

 lates basal metabolism. Deficiencies in thyroxin result in well- 

 recognized abnormalities. Cretinism is due to failure of the thyroid 



Fig. 158. — Photograph of a section through the thyroid of the -^Jjater snake, showing 

 follicles of various sizes'filled with colloid. The trachea is shown at the top of the 

 figure. 



to develop, and individuals thus affected are known as "cretins," 

 many being idiots, undersized, and dependent. In some cases, 

 daily administration of thyroxin to infant cretins results in normal 

 develo])ment. Myxedema is brought about by atrophy of the 

 gland of an adult, as a result of which there is o\'eri)roduction of 

 connective and adipose tissue, slowing down of the heart-beat, 

 weakened ])ulse, thickening of the skin, and dulling of the intellectual 

 powers. Hyperthyroid, on the other hand, ])roduces a condition in 

 which there is a speeding up of basal metabolism, tendency to ingest 

 much food, and an increase in rate of heart-beat. Investigations 

 indicate a functional interconnection between the pituitary and 

 thyroid glands. 



17 



