704 SPECIAL PHYSIOLOGY. 



would taint the nutritive plasma, and cause inflammation, as in reality 

 occurs in rheumatism and gout. 



In the embryo, and in the growing animal, nutrition not only repairs 

 the constant waste of the tissues, but, as already stated, contributes 

 to the formation of new morphological elements, in the processes of 

 development and growth. But, after the body has attained its ma- 

 turity, growth ceases in most, though not in all of the tissues. Hence 

 two kinds of nutritive processes are noticeable in the adult. 



In one, not only are the already existing cell-elements, and their 

 secondary intra- or intercellular products, supplied with materials for 

 their special nutrition, until they have passed through all the meta- 

 morphoses peculiar to them ; but new cell-elements or germinal cen- 

 tres, are constantly being reproduced and developed, for the purpose 

 of supplying the place of those which are cast off; these new cells, 

 are, in turn, succeeded by others. This process, named continuous 

 growth, occurs in the epidermis, nails, and hair, in the epithelial tis- 

 sues of the mucous membranes and secreting glands, and probably also 

 in the gray nervous substance. Moreover, from the active internal 

 changes of absorption and deposition constantly going on in bone, as 

 indicated by observations on the bones of animals fed with madder, 

 it would seem that new cells are continually being formed in that tis- 

 sue, arid, if so, perhaps in cartilage also. In the nutrition of the 

 blood, there occurs not merely a continued renovation of pre existing 

 red corpuscles, but also the death and disappearance of a certain pro- 

 portion of these, together with the reproduction of new ones. In the 

 other mode of nutrition, when once a tissue has attained maturity, no 

 new morphological elements are added to it ; such is supposed to be 

 the case with the areolar and fibrous tissues, and with the muscular 

 and nervous fibres. In these tissues the nutritive process consists 

 merely in interstitial disintegration and deposition, affecting the ele- 

 ments of the perfectly formed tissue, molecule by molecule, so as 

 effectually to preserve the shape and size of a part or organ, as long 

 as the normal or healthy standard of nutrition is maintained. Ac- 

 cording to this view, even the enlargement of a muscle from exercise, 

 is owing to an increase in the size of the fibrillse in each fibre, and 

 not to the formation of new fibres, or even of new fibrillse ; whilst 

 muscular emaciation is due merely to a diminution in the size of those 

 elements. It is, however, supposed by some, that, as in bone, so in 

 muscle, new centres of growth or nuclei are developed from time to 

 time, and give rise to new fibres, amongst the pre-existing ones, some 

 of which are, on the other hand, constantly undergoing retrograde 

 changes, and disappearing. 



In the mode of nutrition by the continuous growth of new cells on 

 the surface, the old elements are cast oft' directly at the surface of the 

 body, or of some one of its internal membranes ; parts of them, how- 

 ever, are sometimes reabsorbed as secretions. In the interstitial mode 

 of nutrition, the products of disintegration, arising from nutritive 

 changes in the substance of the tissue, are taken up, if not by the ab- 

 sorbents, at all events by the bloodvessels, and so enter the blood. 



The phenomena of nutrition are necessarily affected through the 



