230 



a an extreme artery, b b secerning ex- a b 



tremities,c the commencementofa vein, \ \ 



which carries on that blood, peculiar i ^ - 



to this order of structure, and which " ' - 



has been subjected to the function of /' 



secerning tubes which immediately 



precede the venous origins. If the 



arterial structure is continued further than this point, before it 



joins a vein, then, to a corresponding extent, the artery carries 



venous blood, as an artery is known to do in the instance of the 



pulmonary artery, &c. Whether therefore the venous structure 



commences immediately where the secerning occurs, or at a short 



space further than this point, is a matter upon which we cannot 



satisfy ourselves by observation, and upon which, as it is totally 



unimportant, we may be allowed to indulge either conjecture. 



24. The history of the secerning function of the extremities 

 of arteries is still further to be traced. It is to be inquired, what 

 is the process by which the nutrient fluid, whether oxygenated 

 lymph or any thing else, is produced from arterial blood? We 

 cannot answer this question: we can attempt it with an eye to 

 truth only in the analytical way; and what sort of an analysis can 

 we make of properties whose existence is only inferred from their 

 effects of those of the spiritual kind, which are not objects of the 

 senses, and which we cannot denominate? We must be satisfied 

 with referring thif separation to a vital process; and if we would 

 push our investigation into its laws, it is to be pursued by that 

 mode of examining dependences many times described, with a 

 view to ascertain whether the life engaged in this process is the 

 assimilating, or the regular dependent, &c. 



25. The fluid produced by the secerning extremities is con- 

 veyed through the extent of their tubes; and then, becoming ex^- 

 travasated, its relation with the maintenance of life and the tex- 

 tures begins. As the minutest particles of the textures cannot be 

 cylindrical, so if it is necessary for their own regeneration that 

 a nutrient fluid should obtain contact with them, this can take 

 place only by interstitial absorption. Such an absorption implies 

 that solid particles do not touch each other, but are separated by 

 the medium of nutrition; or, if their contact is actual, that it sub- 

 sists only as at a mathematical point. To conceive, however, the 

 process in this way, is perhaps but straining the imagination un- 

 necessarily. Rather than these infinitely minute particles, it may 

 be more convenient to conceive them as distinct molecules of visi- 

 ble bulk connected together, but having irregular interstices, by 

 which a fluid is allowed to pervade their aggregate masses. Thus 

 far we have traced, theoretically indeed, the nutrient material 

 through all its several stages, up to that point which may be said 

 to be its scene of action ; or where the ^Maintenance of organic 

 life, &c. begins: our peroration at this stage is thus briefly made; 

 a nutrient fluid slowly pervades the minutest perceptible molecules 

 of the structures* 



