552 A MANUAL OF PHYSIOLOGY 



leave no unpleasant effects behind them. But it should be 

 remembered that there is no stimulant which is not liable to be 

 abused. It has been shown by ergographic experiments (p. 649) 

 that, like alcohol, tea, coffee, mate, and cola-nut, which all 

 contain the alkaloid theine or caffeine, restore the power of per- 

 forming muscular work after exhaustion, but only if food has 

 been recently or is simultaneously taken. 



Certain organic acids contained in fresh vegetables, although 

 neither in the ordinary sense foods nor condiments, seem to be 

 necessary for the maintenance of health, for in circumstances in 

 which these cannot be obtained for long periods, scurvy is liable 

 to break out. It is prevented by the use of lime or lemon- 

 juice, in which citric and a trace of malic acid are contained. 



INTERNAL SECRETION. 



It is long since Caspar Friedrich Wolff expressed the idea that 

 1 each single part of the body, in respect of its nutrition, stands 

 to the whole body in the relation of an excreting organ,' and 

 thus emphasized the importance of substances produced by the 

 activity of one kind of cell for the normal metabolism of another. 

 But it is only in recent years that it has become possible to 

 illustrate this mutual relation by any large number of experi- 

 mental facts. 



Certain of the substances taken in from the blood by the 

 liver find their way, after undergoing various changes, into the 

 biliary capillaries, and are excreted as bile ; certain other sub- 

 stances, such as sugar and the precursors of urea, are taken up 

 by the hepatic cells, transformed and sometimes stored for a 

 time within them, and then given out again to the blood. Bile 

 we may call the external secretion of the liver, glycogen and 

 urea constituents of its internal secretion. In one sense it is 

 evident that all tissues, whether glands in the morphological 

 sense or not, may be considered as manufacturing an internal 

 secretion. For everything that an organ absorbs from the 

 blood and lymph it gives out to them again in some form or 

 other except in so far as it forms or separates a secretion that 

 passes away by special ducts. But it is usual to employ the term 

 only in relation to organs of glandular build, whether provided 

 with ducts or not. For convenience the action of extracts of 

 some other tissues, such as nervous tissue, will also be con- 

 sidered here, although there is no reason to suppose that they 

 form any specific internal secretion. 



The capacity of manufacturing internal secretions of high 

 importance can neither be attributed to all glands with ducts 



