552 OF NUTRITION. 



consists rather in the wrong plan (so to speak) on which the new matter is laid 

 down, than in a perversion of the nutritive process itself. 



592. Of the mode in which the substitution of new tissue for that which has 

 become effete, is effected in the process of Nutrition, our knowledge is at present 

 limited ; but there can be little doubt that it nearly always takes place in a 

 manner closely conformable to the first development of each tissue. In some 

 instances there is an obvious replacement of the old and dead by the young and 

 active elements : this is the case, for example, in the constantly-repeated pro- 

 duction of the Epidermic and Epithelial layers ; for whether they are developed 

 from germs imbedded in the subjacent basement-membrane, or from nuclei 

 formed de novo in the blastema on its free surface, or by the duplicative subdivi- 

 sion of pre-existing cells, there is a continual succession of new cells, which take 

 the place of those that are cast off as defunct and useless. So in the growth of 

 Hair, the increase of which takes place only at its base, we can trace at any 

 period the same development of newly formed spheroidal cells into horny fusi- 

 form fibres, as that which took place when first the hair began to sprout from the 

 aggregation of epidermic cells at the bottom of its follicle. So, again, in the 

 vesicular tissue which constitutes the essential part of the Nervous centres, 

 there are appearances which indicate that its peculiar cells are in a state of 

 continual development, newly formed ganglionic vesicles taking the place of 

 those which have undergone disintegration. But there are other textures whose 

 nutrition is more completely interstitial; their elements being more closely 

 coherent, and their newly-formed portions being developed throughout the sub- 

 stance of the old, instead of (as in the case of the epidermis and its appendages) 

 superficially or in mere continuity with it. Such is the case, for example, with 

 Muscle, the mode of whose nutrition has not yet been elucidated. We can only 

 infer from analogy, that here too each fibre or fibril will pass, in the course of 

 its development, through the same stages which those of the embryo did when its 

 muscles were first formed. And this analogy seems to derive confirmation, from 

 the presence, in all well-nourished muscles, of bodies which bear the appearance 

 of nuclei ; for these, as Mr. Paget remarks, " are not the loitering impotent re- 

 mains of embryonic tissue, but germs or organs of power for new formation." 

 And it is further confirmatory of this view, that losses of substance of muscle, 

 which involve the destruction of these centres of nutrition, are not replaced, like 

 losses of cuticle, by new tissue of the same kind j the power to form it not being 

 inherent in the blood or in the neighboring parts. Nevertheless, it must be 

 admitted that no intermediate stages of development can be traced in the fibres, 

 even of those muscles of the adult which are in most constant use, and of which 

 the nutrition is the most active, that are at all comparable to those which are 

 met with in the muscular tissues of the embryo. With regard, again, to the 

 interstitial nutrition of Bones and Teeth, we know nothing whatever. That 

 some nutritive change is continually taking place in them, is certain from the 

 fact that if the supply of blood be withdrawn, the parts thus affected die and 

 are cast out of the body ; and it is also made apparent by the effects of madder in 

 gradually tinging even the bones and teeth of the adult, though for such a 

 change a much longer period is required in the adult than in the young animal. 

 The nutrition of the simple Fibrous tissues would also appear to take place 

 interstitially ; but there is no proof that the pre-existing tissues are here in any 

 way concerned in their replacement, which probably takes place in virtue of the 

 self-developing powers of the blood itself. 



593. Of the modes in which the effete particles of tissues whose term of life 

 has expired, or whose vital energy has been exhausted, are removed and disposed 

 of, our present knowledge is no less imperfect. In the case of those tissues 

 which are superficially nourished, a continual loss of substance is obviously 

 taking place by the exuviation of dead particles en masse ; this is the case with 



