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CHAP. VII. Secretion. 



1. THE secreted fluids are generally produced from 

 arterial blood : there is one, the biliary, which is said to be formed 

 from venous blood. Again, secretions are the products of glands 

 and of secerning extremities, and of surfaces ; and they are natural, 

 or the result of disease. 



2. It seems proper first to decide from what blood the secre- 

 tions are produced? If it is settled that secerning extremities of 

 arteries are in general given off, before those tubuli communicating 

 with the veins; it may be inferred, on grounds of analogy, that 

 secretions in general (being formed by secerning extremities) are 

 obtained from arterial blood, because they are produced from 

 vessels, the fluid of which has not yet acquired the character of 

 venous blood. Thus far we are furnished with an argument of 

 analogy, if the grounds of the analogy have been sufficiently ascer- 

 tained. I am, however, acquainted with no certain proof that the 

 secerning system of a gland is given off before the terminations 

 of the arteries in veins. 



3. It seems probable (and yet the foundation for the inference 

 is in some respects questionable) that secretion in general takes 

 place from arterial blood. The results of injections, which are 

 known to favour the conclusion to some extent, alone can prove it ; 

 but of these I am not satisfactorily informed. All the other pro- 

 cesses of the living state appear to require the presence of oxy- 

 genated blood ; and upon this general fact we may, nothing appear- 

 ing to the contrary, infer that the relations engaged in secretion, 

 which is an animal process, acknowledge the same law. 



4. It cannot be determined whether the secretion of a gland 

 is produced from arterial or venous blood, by a ligature upon the 

 communicating trunks. If, for example, the renal artery were tied, 

 the secretion of urine might be suspended merely because the part 

 is no longer supplied with blood of any kind ; and if the vein were 

 tied, the ligature would be upon the trunk, while, if there were any 

 venous branches which gave off secerning extremities, less than, 

 themselves, they must, as may be witnessed by observation, be 

 those which are contiguous to the venous origins. 



5. That, however the secretions take place from tubes which 

 are given off previous to the terminations in veins, it seems war- 



