450 SECRETION. [CHAP. xxxn. 



able measure themselves reabsorbed with the food, so as to be 

 further serviceable for a time in the ever-moving circle of the 

 functions of vegetative life. There is no waste here ; and we may 

 suppose that they do not become finally expelled until they have 

 been reduced to forms of combination, in which they can no 

 longer minister to the life of the tissues. Such secretions are 

 conveniently classed together as recrementitious. 



But it would be an error to suppose that all matters separated 

 from the natural surfaces of the body fall exclusively under one of 

 the foregoing heads; on the contrary, it happens in some cases that 

 a secreted product is of the mixed kind, partly excrementitious and 

 partly recrementitious partly rejected as needless or injurious 

 partly thrown off that it may subsequently fulfil a useful purpose 

 in the oeconomy of the individual and species. Thus the bile con- 

 tains certain matters which are expelled with the excrements, while 

 a large proportion of its elements is reabsorbed into the blood, after 

 their commixture with the chyme in the intestines, and thus fur- 

 nishes material for the production of animal heat in respiration. 



The chief secretions serving an ulterior purpose , and which are 

 thrown out with that object, are the following : The generative 

 elements the milk, the salivary, gastric, pancreatic, and allied 

 fluids ; part of the biliary fluid ; the mucus from some surfaces ; 

 the epidermis and its appendages from the skin; the sebaceous and 

 odoriferous matters from certain glands; the tears. 



Water, as it forms a necessary part of the living frame, and is 

 probably in constant course of formation within it, and as it is, 

 besides, continually received in large quantities as food and drink, 

 is a constant ingredient of the secretions, being thrown off espe- 

 cially by the lungs, skin, and kidneys. By the two former its loss 

 is determined, in a great degree, by simple evaporation into the 

 surrounding air; and this is necessarily influenced much by the 

 hygrometric state and other conditions of that medium. By the 

 latter, whose office is complemental of that of the preceding, a 

 special apparatus is furnished for draining off the water, while this 

 fluid is made useful in extracting the ingredients of the most im- 

 portant of the excretions from the surface over which it is subse- 

 quently made to flow. 



Vicarious Secretion. It has been remarked, that there is a sort 

 of compensating action between the skin and kidneys in the 

 normal condition of the system, dependent upon variations in tem- 

 perature and other conditions. A similar power exists, in a more 

 limited extent, in other secreting structures, by virtue of which 



