90 Comparative Aiiatomy ; or an Attempt at 



Having now described those parts of the human body and 

 vegetable frame, o\\ which their figure, support, strength, and 

 motion depend ; we must next examine those juices which are 

 intended to replace the waste of both the machines, and to sup- 

 ply them with new energies. In the animal body are many more 

 sorts of liquid than in the vegetable, because these are so many 

 Eecretions from the blood: nov/ this is unknown in plants. The 

 animal blood is a rich store of nutritious fluid, line enough to 

 penetrate its minutest parts, and which constantly circulates 

 through its whole machine : the elemer.ts of its composition are 

 furnished by all our food. Now in the vegetable there are but 

 tivo sorts of liquid that can be said to replace the waste of its 

 substance, — the bark-juice and sap: the fii^t, far from being 

 formed in the body, is merely collected there from the atmosphere 

 bv the hairs, in one of the layers of the leaf, from which place 

 it runs to the inner bark-vcbsels. It has no circulation like the 

 blood; nor is it formed by the food, but taken in when separate, 

 and mixed in the pabulum of the leaf. The bark -juices are so 

 far from being capable of circulating, that their tendency to co- 

 agulate is such, that the bark is full of large species of matter 

 escaped from its licpiid stale ; and even in the vessels of the inner 

 bark (if it was not for their peculiar cnnstructioii) it would never 

 retain its liquid form, since it is forced through apertures not 

 half the size of the vessels in which it runsj and this impelus, by 

 increasing the violence of the act inn, tends to lis fluidity. The 

 sap is, however, better adapted to play the part of the blood, 

 were not a circulating medium totally unfit and unsuilcd to the 

 vegetable form. A being increasing at each of its numerous ex- 

 tremities, receiving its blood from another body, and that Hood 

 coagiilfiling iiilo^ a jelly by rest and suspension, before it can 

 form its compound, — can such a make be compatible with a coi>- 

 stant circulation of blood ? which cannot stop without destroying 

 life. The proving that the sap-juices do not circulate is of the 

 greatest consequence: if they circulate, and all the juices that 

 should (when tliev arrive at the extremities) instead of forming 

 the new shoot, run down again through the bark, — how is the 

 new shoot to be completed? since all that matter which was to 

 form it is employed in returning through the bark, leaving the 

 only part whicli at that time required support and moisture. Is 

 it not more natural to suppose that the juices (as they always 

 become a jelly before they are wood) are arrested to be converted 

 into that matter, and then complete their change? The idea of 

 circulation is drawn from the comparison of animal life, j//?^ only 

 in ajeiv instances, misleading in ahiiost every other. Is there 

 not a great difference between an animal which grows not at its 

 extremities, but which after the fi>st k\y years ceases to inr roase, 



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