OF SECRETION IN GENERAL. 507 



ture. This may be partly accounted for by the imperfect degree in which, so 

 long as the entire organism is undergoing rapid increase, the normal structure 

 is developed in any one portion of it; for the degree of consolidation bcin^- 

 less, the tendency to decay will naturally be greater. But this explanation is 

 not in itself sufficient ; and we must be content for the present to regard it as 

 a general law (which may ultimately prove to be but a result of some more 

 general principle), that the duration of the existence of individual cells in- 

 creases, cseteris paribus, with the advance of life. At the same time, their 

 functional activity diminishes. They may be said to live more slowly. The 

 dull perceptions, and slow and feeble movements of the aged man, form a 

 striking contrast with the acute sensibility, and the rapid and vigorous mus- 

 cular actions of the child ; and the same change may be noticed in the organic 

 functions. Hence it may be stated as a general law, that the vital activity of 

 the Cells (and of the tissues produced by their transformation) diminishes in 

 proportion to the prolongation of life ; and this law exactly corresponds with 

 what has just been observed as to the comparison of the tissues of different 

 kinds, which are present in the same body. 



CHAPTER XII. 



OF SECRETION. 



I. Of Secretion in General. 



647. THE literal meaning of the term Secretion is separation ; and this is 

 nearly its true acceptation in Physiology. We have seen that the Nutritive 

 materials, which are received into the living body, are combined in a certain 

 proportion in the circulating fluid ; and that they are carried in its current to 

 every part of the structure. A part of the elements of the Blood, probably 

 the Fibrin and mineral matter exclusively, are being continually separated 

 from it, and introduced into the solid textures, of which they become consti- 

 tuents ; forming, as it were, the organized framework, in the interstices of 

 which various other matters (also separated from the blood) are deposited in 

 an inorganic condition. This separation, the object of which is to build up a 

 living fabric, has been already considered under the head of Nutrition ; but it 

 may be here remarked, that the deposition of Calcareous matter in the Bones 

 and Teeth, of Chondrin and Gelatin in the Bones and Cartilages, and of Horny 

 matter in the cells of the Epithelium and its appendages (Hair, Nails, Hoofs, 

 &c.), is accomplished by a process analogous in all respects to that concerned 

 in the separation of those other products which are ordinarily considered as 

 Secretions. The same may be said of the Serous fluid, which distends the 

 interspaces of areolar tissue, the Oily matter contained in the Fat-cells, the 

 Albuminous fluid of the Humours of the Eye, and other analogous constituents 

 of the living fabric. But we have chiefly to consider under the present head, 

 the nature and origin of these products, which are continually being cast forth 

 from the living body ; the amount of which is usually, in the adult animal, 

 equal to that of the solids and fluids ingested, after allowance has been made 



