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LECTURE XIV. 



ON THB PE0CES3 OF ASSIMILATION AND NUTRITION ; AND THE CURIOUS 

 EFFECTS TO WHICH THEY LEAD. 



We have traced out in our preceding studies something of the means 

 by which form, and magnitude, and motion, are produced in the inorgan- 

 ized world : — how the various substances that surround us combine and 

 separate, vanish from us and re-appear, and, in the multifarious processes 

 they undergo, give rise to new products by new and perpetually shilling 

 involutions. We have further traced an outline of the means by which 

 organized matter is capable of building up the curious structures of plants 

 and animals ; how the chief functions they possess are carried on, and by 

 what means they respectively acquire maturity and perfection. 



But it is not only necessary that the system should in this manner be 

 matured and perfected by a fresh application of materials, but that the old 

 materials which constitute every organ should be progressively removed 

 firom the system, in consequence of their being worn out by use, and their 

 place supplied from definite stores. Let us, then, devote the present hour 

 to an inquiry how this latter change occurs in vascular and living matter, 

 in the vegetable and animal system : by what means the dead or exhausted 

 and worn-out elements of the different organs are carried of!', and replaced 

 by new reformative materials, and what are the principal pha^nomena that 

 result from such a series of operations. 



The blood, then, in animals, and the sap, which may be regarded as a 

 species of blood, in plants, of both which we have already treated, are 

 the vital currents from which every organ of the individual frame derives 

 the nourishment it stands in need of, and into which it pours ultimately a 

 considerable portion of its waste and eliminated fragments ; for the pro- 

 vident frugahty of nature suffers nothing to be lost, and, as far as possible, 

 works up the old materials, time after time, into fresh food for the sub- 

 sistence of the entire system. 



To produce this double purpose two distinct sets of vessels are neces- 

 sary : one for that of separating from the common mass of the Wbod, and 

 re-combining into new associations, those particular parts of it which the 

 formation of the fresh matter demands ; and the other for that of carrying 

 back the rejected materials into the general current, And hence these 

 two sets of vessels bear the same relation to each other as the veins and 

 arteries of the animal frame, accompany every part of the frame to its far- 

 thest extremities, and, indeed, constitute the general mass of the frame 

 itself. From the respective offices they perform, they are denominated 

 SECERNENT and AKsoKBENT systcms ; in their utmost ramifications they 

 are too minute to be traced by the keenest eye, or the nicest experiment 

 of the anatomist ; but, where they are not quite so minute, they are suf- 

 ificiently discoverable, and their course is sufficiently capable of being fol- 

 lowed up, from the dehcate apertures or mouths by which, in iiifinite 

 numbers, they open on all animal surfaces, or hollows whatever, to their 

 incipient sources. 



The sECERNENTs, or that set of vessels whose office it is to separate 

 particular parts from the blood for particular purposes, are evidently con- 

 tinuations of some of those very subtile ramifications of the arteries which, 



