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them, except that of size, on which we can scarcely presume, we 

 should, from these poiuts of similitude, be inclined to reply, that the 

 assimilation of the spirit belonging to the arterial tubes does not 

 take place from the blood which flows through them. But as tlws 

 analogy, confessedly, is not quite perfect, we will attempt to decide 

 the question of the seat of assimilation by viewing the matter on 

 other grounds. 



5. The component structures of the body may be said, in the 

 gross, to be tubular, fibrous, or consisting of particles, held together 

 by some power of cohesion : into this last, perhaps, the fabric of 

 both the others might be ultimately resolved. Of these, the vascu- 

 lar system is an example of the first; the muscular structure, of the 

 second ; the osseous (with some equivocal exception*), of the third^ 

 6. As we found reason to believe that the heart is not 

 nourished by the blood in its cavities, but by that supplied to it by 

 the coronary arteries, so it is currently said that the arteries also 

 have their nutrient vessels, and that they live by the blood furnished 

 to them by the vasa vasorum. The proofs of the existence of these 

 minute vessels are, that the structure of the arteries contains blood, 

 which it can do only by communicating with the arteries; and that 

 their structure may be injected. These proofs, however, are by no 

 means conclusive; though it is scarcely worth while to shew in this 

 place in what they are defective, 



7. If an artery is nourished by a minuter order of vessels*, by 

 what are these vessels nourished? Will it be replied, by vessels still 

 more minute? this would be rather to strain the imagination. But 

 the existence of these still more minute vessels granted, even they, 

 to be identified as vessels, must have their eoats, their internal and 

 their external surfaces, and their intermediate composition of organic 

 particles. Jf the existence of such tubuli is supposed further than 

 it might be demonstrated, wtere is the supposition of them to end? 

 In truth, the supposition of tubuli may proceed in injimtum? but, 

 happily, it is made obvious that the declension of the area of tubuli 

 has some limit, by the fact ttoat when the smallest tubulus is sup- 

 posed, the lines of constituent particles, of which its sides are com- 

 posed, must be less than the tube itself, Hence we come to the 

 inference, that particles are laid down* which are not directly obtained 

 from continuity ef tubular cavities. 



\ 8. But, independent of this limit to the gradual diminution of 

 tubuli, it is sufficient to assign the existence of tubes, continuous 

 with the arteries, so far as there is evidence of their existence; and 

 it may therefore be said, with the anatomists, arteries terminate in 

 secerning extremities, &c. 



9. Now the questions to be decided are, whether nutrition 

 takes place from the blood in any order of vessels? or whether it 

 takes place from exuded fluids? or whether from both these sources? 

 We can scarcely hope to decide these questions by an evidence 

 amounting to proof; the argument may, however, be stated 

 93 follows: 



