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1st, The life of a targe artery is maintained where the circu- 

 lation of blood through its canal is prevented. Hence, from the 

 similitude subsisting between the arteries, so far as their structure, 

 &c. is capable of being ascertained, it is on this ground of analogy 

 indicated, that the life and nutrition of all orders of these vessels 

 are supported by fluids which do not circulate in their canals. 



2nd, Tbata nutrition of vascular structures takes place from 

 fluids not contained in the blood-vessels, may be positively in* 

 ferred from the fact mentioned above, viz. that the nutrition of 

 component particles of the minutest tubuli cannot take place 

 directly without exudation from a vascular tube. 



3rd, That as an extra-tubular source of nutrition may be said 

 to be a necessary one, so any other mode not supported by testi- 

 mony of the slightest kind is to be treated as an unfounded con- 

 jecture, aad in agreement with rules of reasoning rejected; be- 

 cause it is contrary to the analogy furnished by the fact first cited, 

 and because also it differs from the nutrition ascertaiuable ou 

 other grounds, in these respects, 1st, that the blood which circu- 

 lates in the vessels moves rapidly, while extravasated fluids have 

 only a slow motion, or are nearly at rest; and, 2nd, that the fluids 

 having a nutrient relation with minute particles are extensively, 

 if not universally, products from blood, and not red blood. On 

 these data it appears not too much to assume a general, if not 

 an universal, mode of nutrition, from fluids not contained in 

 blood-vessels. 



10. If the material of nutrition is not contained in vessels, 

 identified by their coats, &c. there is only this alternative, viz. 

 that the nutrient fluids are extravasated, or effused from the se- 

 cerning extremities of the arteries, aud that they obtain contact 

 and relation with the particles of the structures by a porous or 

 interstitial absorption. 



11. We have traced some stages of the preparation of a fluid 

 whose end is nutrition, or the maintenance of life, and the struc- 

 tures, from its state of food, to its formation into blood. It seems 

 consonant with analogy to suppose, that the seceruing extremities 

 of arteries exert a function with respect to blood, which is a fur- 

 ther, or an an additional, slage of preparation, for the above 

 object, to those which we have already enumerated. 



12. That a preparation of blood which is connected with the 

 above purpose taJkes place, is proved by the facts before cited ; 

 and which will be spoken of again, shewing that the products of 

 secerning extremities are, perhaps, in no instances mere filiations: 

 but what relations the changes which blood sustains, by the pro- 

 cesses of secerning extremities, may have with the purposes their 

 results are destined to fulfil, is a question of more especial 

 and difficult investigation. 



13. It may here be urged, that if a bone is scraped it bleeds; 

 and that with such an exudation, as to warrant the supposition 

 that red blood is extravasated, which, it may further be conjee- 



