262 BIOLOGY. [Book it. 



■which we do not find in the blood (peptine, pancreatine, and so 

 on).» 



In reality, the secreting glands are at the same time organs of 

 excretion. All of them take from the blood water and salts, 

 substances to which they ofier a passage without in any respect 

 changing them. But besides they form, at the expense of the 

 sanguineous materials, a special azotised product. Very habitually 

 the agent of these chemical transformations is the epithelial cell. 

 Sometimes, howeyer, the metamorphosis can be accomplished in 

 the wall itself of the gland, as is the case, for instance, with the 

 mammary gland.^ But the secreting element by excellence is 

 the epithelial cell, whose numerous varieties have each their 

 special affinities. 



When neither the wall of the glandular organ, nor the epithelial 

 cells which are contained therein, exercise any modifying action 

 on the materials of the blood, hut fulfil simply the office of a 

 filter, offering a passage to certain substances and refusing it to 

 others, there is merely excretion. Excretion is a biological act 

 more simple than secretion, and comparable with the exhalation 

 which goes on, for instance, on the pulmonary surface. 



The excretory glands are never closed. Always they pour the 

 humour which they filti'ate on some point of the tegumentary, 

 cutaneous, or mucous surface, and this humour is destined 

 to co-operate with no ulterior physiological function. It is a 

 dead product, whose expulsion is necessary, and whose retention 

 in the organism would be fatal. It is the residuum of nutrition. 



The excrementitial humours, of which the sweat and the ui-ine 

 are the types, are solely constituted by the water, holding in 

 solution saline principles, and also crystallisable azotised sub- 

 stances, which, formed in the anatomical elements themselves by 

 disassimilation, pass fii-st of all into the blood, whence they are 

 extracted and excreted by the glands. 



1 Ch. Robin, Lefons mr Us Humeurs Normales et MorUdes, pp. 29, 30, 32, 

 and Introduction, p. xxvi., xxvii. 



2 Ch. Robin, loc. cit., pp. 17, 18. 



