Innutrition of the textures. 17 



ceivable mode in which the renoyation of matter might be brought 

 about, namely, by a molecular change which renews the substance, 

 particle by particle, without affecting the form or structure ; by a pro- 

 cess, in short, which might be termed " molecular renovation." Still, 

 although conclusive evidence is wanting on the point, it seems probable 

 that the crude material of nutrition first undergoes a certain elaboration 

 or preparation through the agency of cells disseminated in the tissue ; 

 which may serve as centres of assimilation and increase, as already 

 explained. 



Office of the vessels. — In the instance of cuticle and epithelium, no 

 vessels enter the tissue, but the nutrient fluid which the subjacent 

 vessels afford penetrates a certain way into the growing mass, and the 

 cells continue to assimilate this fluid, and pass through their changes at 

 a distance from, and independently of, the blood-vessels. In other 

 non-vascular tissues, such as articular cartilage, the nutrient fluid is 

 doubtless, in like manner, conveyed by imbibition through their 

 mass, where it is then attracted and assimilated. The mode of nutri- 

 tion of these and other non-vascular masses of tissue may be compared, 

 indeed, to that which takes place throughout the entire organism in 

 cellular plants, as well as in polypes and some other simple kinds of 

 animals, in which no vessels have been detected. But even in the 

 vascular tissues the case is not absolutely different ; in these, it is true, 

 the vessels traverse the tissue, but they do not penetrate into its 

 structural elements. Thus the capillary vessels of muscle pass between 

 and around its fibres, but do not penetrate their inclosing sheaths ; 

 still less do they penetrate the fibrillfe within the fibre ; these, indeed, 

 are much smaller than the finest vessel. The nutrient fluid, on exuding 

 from the vessels, has here, therefore, as well as in the non-vascular 

 tissues, to permeate the adjoining mass by transudation, in order to 

 reach these elements, and yield new substance at every point where 

 renovation is going on. The vessels of a tissue have, indeed, been not 

 unaptly compared to the artificial channels of irrigation which distri- 

 bute water over a field ; just as the water penetrates and pervades the 

 soil which lies between the intersecting streamlets, and thus reaches 

 the growing plants, so the nutritious fluid, escaping through the coats 

 of the blood-vessels, must permeate the intermediate mass of tissue 

 which lies in the meshes of even the finest vascular network. The 

 quantity of fluid supplied, and the distance it has to penetrate beyond 

 the vessels, will vary according to the proportion which the latter bear 

 to the mass requiring to be nourished. 



We have seen that in the cuticle the decayed parts are thrown off" at 

 the free surface ; in the vascular tissues, on the other hand, the old or 

 effete matter must be first reduced to a liquid state, then find its way 

 into the blood-vessels, or lymphatics, along with the residual part of 

 the nutritive plasma, and be by them carried off". 



From what has been said, it is clear that the vessels are not proved to perform 

 any other part, in the series of changes above described, beyond that of convey- 

 iag matter to and from the scene of nutrition ; and that this, though a necessary 

 condition, is not the essential part of the ijrocess. The several acts of assuming- 

 and assimilating new matter, of confemng on it organic structure and form, and 

 of disorganising again that which is to be removed, which are so many manifes- 

 tations of the metabolic and plastic properties already spoken of. are performed 

 beyond the blood-vessels. It is plain, also, that a tissue, although devoid of vessels, 

 VOL II. c 



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