CHAPTER XVI 



INTERNAL SECRETION THYROID AND 

 SUPRARENAL GLANDS 



THE progress of physiology in the last few years has shown 

 that it is impossible to assign any one special function to any 

 organ. The different organs of the body have a complex inter- 

 action and work together for the good of the whole organism, 

 which is the physiological unit. Nowhere is this interaction 

 better shown than in the case of the thyroid, suprarenal, pancreas 

 and the generative glands. The ductless glands produce a secretion, 

 " the internal secretion," which is absorbed by the blood-vessels or 

 lymphatics ; glands with ducts and an obvious secretion produce 

 also other substances of the nature of an internal secretion which 

 is necessary for the health of the animal ; the thyroid and suprarenal 

 glands will be taken as types of the former class, the pancreas 

 belongs to the latter class, and its functions are discussed in another 

 part of this work. 



The recent investigations upon these glands have shown the 

 futility of attacking such problems of life from an anatomical or 

 morphological point of view. Purely anatomical reasoning in such 

 cases leads astray. Some of the glands in question are so small 

 that it was considered improbable that they were essential for 

 the welfare of the animal ; others were shown by the study oi: 

 their morphology to represent organs more fully developed in 

 lower animals, and it was therefore concluded that in the higher 

 animals they were present only as rudiments, more or less useless 

 relics. Whether we believe in Design or in Natural Selection during 

 the struggle for existence, it is a useful guide to believe that every 

 part of the organism is of some use ; life is conducted on economical 

 lines. 



Thyroid Gland. The function of this gland must now be con- 

 sidered in detail. Most of our knowledge has arisen within the 

 last thirty or forty years. Previously to that time many fanciful 



