THE DEEPENING OF PHYSIOLOGY. 291 



Of each of these steps we propose to give some brief 

 illustration. 



(1) Elucidation of Enigmatical Organs. In the 

 body of a higher animal there are numerous organs 

 which take materials from the blood and get rid of 

 these, usually in modified form, as a secretion which 

 exudes through a duct or ducts on some internal or 

 external surface. We call these "glands"; the 

 liver, the pancreas, the sweat-glands, the milk -glands 

 are familiar examples. 



But there are other organs, somewhat analogous in 

 structure, which though they take materials from 

 the blood, and form a secretion, have no ducts. If 

 these " ductless glands " get rid of their secretion 

 it must be by returning it to the blood. Some of 

 them have directly to do with the cells of the blood ; 

 thus the spleen is in mammals a grave for worn-out 

 red blood corpuscles, while in some lower verte- 

 brates it seems to be one of their birthplaces. But 

 in many other cases the ductless glands do not return 

 any cellular material to the blood, i.e., they do not 

 form corpuscles, and what fluid material they return 

 to the blood can only be discovered indirectly. A 

 good example of this is furnished by the thyroid 

 gland. 



The thyroid gland is a small reddish organ, richly 

 supplied by blood-vessels, weighing from one to two 

 ounces in man, situated in the front of the throat on 

 each side of the windpipe. What its precise function 

 is we do not yet know, but very suggestive hints have 

 been gradually accumulating of recent years, and 

 we are certain that in spite of its minuteness it is 

 extremely important. When it atrophies or is ex- 

 cised, the disease myxoedema ensues, in which the 

 connective tissue becomes overloaded with mucinous 



