614 OF SECRETION. 



CHAPTER XV. 



OF SECRETION. 



1. Of Secretion in General. 



817. The literal meaning of the term Secretion is separation; and this is 

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

 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. Of the elements of the Blood, some are being 

 continually separated from it, to be introduced into the solid textures, of which 

 they become constituents ; forming, as it were, the organized frame-work, 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 Chondrine and Gelatine in the Bones and 

 Cartilages, and of Horny matter in the cells of the Epithelium and its ap- 

 pendages (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. 



818. But we have chiefly to consider under the present head, the nature 

 and origin of those products which are continually being cast forth from the 

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

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

 portion rejected, in the form of faeces, as indigestible. The experiments of 

 Dr. Dalton* on his own person, give the following as the proportional quan- 

 tities discharged through the principal channels of excretion. The mean 

 quantity of solid and liquid Aliment taken into the system daily (during 14 

 days in spring) being 91 oz., or about 5| Ibs., the average amount of Faeces 

 (including part of the solid matter of the bile) was 5 oz. ; the average amount 

 of Urine was 48^ oz. daily; and, as the total weight of the body remained 

 the same, the quantity of fluid and solid matter excreted by the Skin and the 

 Lungs must, have been 37.i oz. At other periods of the year, a variation was 

 observed ; especially in the relative amount of fluid passing off by the Urine, 

 and by Cutaneous exhalation. 



819. It can scarcely be questioned, that the chief source of the Excretions 

 is to be found in the continued Decomposition of the various tissues of the 

 body, which has been several times alluded to ( 275 and 811); and it is pro- 

 bable, from considerations heretofore adduced, that they are derived, not so 

 much from the fluid returned into the blood by the Lymphatics (as formerly 

 supposed), as from the Blood itself ( 680). It has been pointed out by Lie- 

 big, that there is a remarkable correspondence between the elements of the 

 Blood, and those of the Bile and Urine taken together ; so that the Tissues, 



* Edinburgh New Philosophical Journal, 1832, 1833. 



