712 AN^iUAL EEPOET SMITHSONIAN INSTITUTIOIT, 19 2 8 



the other hand, the frog is unable to alter the color scheme of its skin 

 when the illumination of its surroundings is altered. There is little 

 doubt, then, of the action of this portion of the pituitary upon the 

 pigment cells of the frog. If the eyes of a normal frog are covered 

 with some opaque material, the color reaction does not occur. 



Now, we can form a picture of the mechanism by which the animals 

 change their tinting according to the light. If the frog is in a bright 

 light or upon a light background, the retina is stimulated by the 

 rays of light. Impulses pass along the optic nerves to the brain, and 

 some of these impulses are sidetracked to the pituitary's posterior lobe 

 and its secretion is suppressed. The suppression persists so long as 

 the rays of light are entering the eyes. The pigment cells, however, 

 do not at once become pale, for the secretion which had been circulat- 

 ing in the blood, and which keeps the cells expanded, must first be 

 used up. This takes a certain time, but as soon as it is complete the 

 cells appear to shrink, and a sickly greenish-yellow pallor creeps over 

 the amphibian skin. In this way does the frog, leopard, or other 

 variety, change his spots ; but the Ethiopian can not change his skin, 

 for no corresponding reactions have been shown to occur in man 

 or in anj^ of the higher animals. 



Limited space permits me to consider only two ductless glands. I 

 have chosen these two because more is known of them than of others 

 which I have not touched upon. I might, for instance, have discussed 

 the parathyroids, those tiny glands which lie beneath the thyroid. 

 They are essential to health, and indeed to life itself, though even in 

 an ox they are no larger than a bean. They govern the amount of 

 calcium in the blood. If they are removed, the calcium is reduced, 

 and an extract prepared from them when injected into an animal 

 causes the calcium to rise. 



Or I might have taken up the adrenal glands, which lie near the 

 kidneys and are believed by some to enhance the reactions of the 

 primitive emotions, hate, fear, anger, etc. There are also those glands 

 which have been shown to exert such influence upon the development 

 of the secondary characteristics of sex, such as the plumage of birds 

 and the antlers of stags. There are also the pineal gland and the 

 thymus, the one Ij'ing deep in the brain, the other deep in the chest, 

 and both shrouded in the deepest mystery. There is also the pan- 

 creas, which pours insulin into the blood stream, but the study of this 

 hormone is a very large subject in itself. 



I have treated the glands of which I have spoken as though they 

 were quite independent one of the other. This is unavoidable, be- 

 cause this is the way in which they have been studied, and this is the 

 way in which most of the information regarding them has been 

 gained. Yet there is no doubt that their actions are very closely 

 related one with another and that it is purely artificial to study them 



