482 A MANUAL OF PHYSIOLOGY 



(7) Recent experience goes to show that in severe and continuous 

 exertion, coupled with exposure to all weathers, as in war and in 

 exploring expeditions, alcohol is injurious, and it is well known that 

 it must be avoided in mountain climbing. 



Tea, coffee, and cocoa are more suitable stimulants for healthy 

 persons, because they are less dangerous than alcohol, and they 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. 621) that, like alcohol, 

 tea, coffee, mats', and cola nut, which all contain the alkaloid theine 

 or caffeine, restore the power of performing 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 '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 experimental 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 

 substances, 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 manu- 

 facturing 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 



