354: COMPARATIVE PHYSIOLOGY. 



undertake an unusual share of excretory work, probably even 

 to tbe length of discharging urea. 



Obscure as the subject is, and long as it may be before we 

 know exactly what and how matter is thus excreted, we think 

 that it will greatly advance us toward a true conception of the 

 vital processes of the mammalian body if we regard the ali- 

 mentary tract as a collection of ^organs with both a secreting 

 and excreting function ; that what we have been terming ab- 

 sorption is in the main, at least, essentially secretion or an 

 allied process ; and that the parts of this long train of organs 

 are mutually dependent and work in concert, so that when one 

 is lacking in vigor or resting to a greater or less degree, the 

 others make up for its diminished activity ; and that the whole 

 must work in harmony with the various excretory organs, as 

 an €xcretor itself, and in unison with the general state of the 

 economy. We are convinced that even as an excretory mech- 

 anism one part may act (vicariously) for another. 



Of course, in disease the condition of the f»ces is an indica- 

 tion of the state of the digestive organs ; thus color, consistence, 

 the presence of food in lumps, the odor, and many other points 

 tell a plain story of work left undone, ill-done, or disordered 

 by influences operating from within or from without the tract. 

 The intelligent physician acts the part of a qualified inspector, 

 surveying the output of a great factory, and drawing conclu- 

 sions in regard to the kind of work which the operatives have 

 performed. 



THE CHANGES PRODUCED IN THE FOOD IN THE 

 ALIMENTARY CANAL. 



We have now considered the method of secretion, the secre- 

 tions themselves, and the movements of the various parts of 

 the digestive tract, so that a brief statement of the results of 

 all this mechanism, as represented by changes in the food, will 

 be appropriate. We shall assume for the present that the effects 

 of the digestive juices are substantially the same in the body as 

 in artificial digestion. 



Among mammals food is, in the mouth, comminuted (except 

 in the case of the carnivora, that bolt it almost whole, and the 

 ruminants, that simply swallow it to be regurgitated for fresh 

 and complete mastication), insalivated, and, in most species, 

 chemically changed, but only in so far as starch is concerned. 



