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He then proceeded to consider the general theory of excretion in 

 the animal body, beginning this subject with the sti'iking fact, noticed 

 by both Prout and Liebig, that although oxygen from the air must 

 be a main agent in forming all the excretions (because oxygen exists 

 in them all in a larger proportion than in those aliments and those 

 textures of the body from which they must be respectively derived), 

 yet the oxygen taken in at the lungs does not appear to enter into 

 the combinations by which the excretions are formed (particularly, 

 does not form carbonic acid), as long as it is passing along the ar- 

 teries, but " changes its mode of action" when it reaches the capil- 

 laries, where it must meet with the matter absorbed from the tex- 

 tures. He stated it as the general, and apparently first, opinion of 

 physiologists, that the excretions are furnished partly by redundant 

 ingesta, and partly by "effete" matter in the system itself; but the 

 important agency of oxygen in maintaining them had not been so 

 generally recognised, and the term " effete" has in general no very 

 definite idea attached to it. But, combining together all that is 

 known as to the continued interstitial absorption in animal bodies, 

 the continual introduction of oxygen into them, and as to the nature 

 and quantity of the excretions, as compared with the ingesta, he 

 stated it as the most general expression of these facts, that through- 

 out all the parts of any living animal where nutrition and absorption 

 are going on, i. e., at the extremities of the capillary vessels, in the 

 more perfect animals, carbon, nitrogen, hydrogen, and oxygen, are 

 continually forming two sets of compounds, — certain portions of 

 these elements, recently introduced in the form of aliments, either 

 separating, in the form of the organic compounds, from the other 

 constituents of the blood, or uniting to form those compounds, — and 

 in either case attaching to themselves particles of earthy and saline 

 and inflammable bodies, taking the form of cells or fibres, and build- 

 ing up the organised frame ; while other portions of the same, which 

 have been for some time in the body, rejecting these adventitious 

 matters, and uniting with oxygen from the air, are continually fall- 

 ing into the proportions by which the compounds destined to excre- 

 tion are formed, which are poisonous to the system if retained, which 

 tend always to the crystalline form, and, in fact, are steps in the 

 process of the gradual restoration of these elements to the inorganic 

 compounds, carbonic acid, water, and ammonia, out of which the 

 agency of vegetable life had originally formed them. 



This general fact he considered as the clearest proof of that princi- 



i 



