OF NUTRITION. 343 



CHAPTER VII. 



OF NUTRITION. 



1. Selecting Power of the Individual Parts. 



612. THE Blood which is carried into the different parts of the 

 system, by the Circulating apparatus, is the source from which all the 

 organs and tissues of the body derive the materials of their growth and 

 development ; and, as we have seen, it is distributed by the Capillaries 

 of the several tissues, with a degree of minuteness, which varies accord- 

 ing to the activity of the nutrient operations taking place in the indi- 

 vidual parts. Thus, in Nerve and Muscle, Mucous Membrane and 

 Skin, a constant decay of the old, and a development of new tissue, are 

 taking place; when these organs are in a state of functional activity ; 

 and a copious supply of blood is carried through every part of their 

 substance: whilst in Cartilage and Bone, Tendon and Ligament, the 

 amount of interchange is very small, and is effected by a much less 

 minute reticulation of capillary blood-vessels. 



613. The materials of the nutritive process being prepared in the 

 blood, the process of nutrition is the act of each individual part ; which 

 grows and developes itself, in virtue of its own inherent powers, as long 

 as the requisite conditions are supplied. The mode in which this takes 

 place, in each individual tissue, has been already- explained in the for- 

 mer part of this Treatise. We have seen that, in the great majority 

 of cases, the act of Nutrition is, in fact, a process of cell-growth ; and 

 that it takes place under the same conditions as the production of the 

 simple isolated cell, which constitutes the whole of the humblest forms 

 of Cryptogamic Vegetation, namely, that it grows from a germ, which 

 draws to itself the materials of its nutrition, and gives to some of them 

 a new arrangement, whereby they form the cell-wall, whilst others are 

 introduced into the cell-cavity,- and that when it has passed through 

 its regular series of changes, it dies, and sets free its contents. We 

 have seen that, in some cases, the germs are prepared by previously- 

 existing cells of the same kind ; whilst in others they are furnished by 

 certain " nutritive centres," which seem to be constantly engaged in 

 the preparation of them, deriving their materials from the blood. Fre- 

 quently it would seem as if the original or parent-cell is able to con- 

 tinue the production of secondary cells to an unlimited extent, even 

 though it may have itself undergone a considerable change of form. 

 Thus the ultimate follicles of Glands seem to be at first closed cells, 

 which subsequently open at the part nearest to the duct, and establish 

 a connexion with it ; and having thus changed their condition, they go 

 on developing new generations of secreting cells in their interior, from 

 their own nuclei or germinal centres, to an unlimited extent. In like 

 manner, the parent-cells of Muscular Fibre, which have coalesced to 

 form the tubular Myolemma, seem to continue to develope new fibrillae 

 from their nuclei, notwithstanding their change of form. 



