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rantable to conclude from this fact, in addition to the general 

 analogy before spoken of, viz. that the circulation in all the per-f 

 ceptible ramifications of veins tends towards their trunks, while 

 the reverse happens with the arteries. 



6. One exception to this remark occurs in the instance of the 

 vena portae : and here, agreeably with the exception, an office of 

 secretion has been inferred, though how justly is indeed, without 

 the aid of a strong case in point, merely from a priori reasoning, 

 extremely dubious. The branches of the vena portaj, instead 

 of collecting towards a trunk from minute beginnings, as is usual, 

 are distributed from a trunk, and are minutely divided in the 

 structure of a glandular viscus. Unless this exception to the 

 ordinary arrangement be admitted as a testimony, I am acquainted 

 with no other proof that bile is secreted from venous blood. 



7. Moral reasoning on physical subjects is sometimes absurdly 

 proposed, where there is a deficiency of other proofs. In this way 

 we might be asked, why should so unusual a distribution of a vein 

 occur in the liver, unless it is subservient to the peculiar function 

 of the organ, to the secretion of bile I Why, in the way of paral- 

 lel it may be asked, did God make men with any livers at all, 

 seeing that as he is omnipotent he might have made them equally 

 well without them? In the latter case, it will be replied, it was 

 God's will that men should have livers; and in the former it must 

 be answered, nature has determined that the distribution of the 

 ven. port, should be different from that of the other veins: if we 

 would know what end is answered by this peculiarity, it is not the 

 mere confession of a peculiarity that will inform us, but we 

 must trace it by an appropriate inquiry. 



8. If mercury would pass from the trunk of the ven. port, 

 into the secerning structure of the liver, and finally make its escape 

 through the gall-ducts, the evidence of such a continuity of tube 

 would indeed be tolerably conclusive on the question; and, for 

 further satisfaction, the experiment might be repeated on the 

 hepatic artery : but as such a proof, although perfectly practicable, 

 has not to my knowledge been attained, I shall allow the matter to 

 rest with these suggestions, stating the argument upon these points 

 thus: there is reason to believe on certain grounds, partly of per-? 

 ceptible phenomena and partly of analogy, that the secretions in, 

 general are formed from arterial blood; there is however one 

 instance conjectured, on weak or questionable grounds, of a secre- 

 tion produced from venous blood; thus indicating that secretion 

 might be produced from either, by a corresponding modification 

 of the powers engaged in the process. 



9. In agreement with the agencies concerned in the pheno- 

 mena of animal bodies, three modes of the process of secretion 

 might be suggested : 1st, that it is a mechanical operation ; 2nd, 

 that it is a chymical one; 3rd, that it is one governed or per- 

 formed by the properties of life. These modes respectively (each 

 having had its advocates) may be briefly considered. 



