508 OF SECRETION. 



for the portion rejected, in the form of faoces, as indigestible. The experi- 

 ments of Dr. Dalton* on his own person, giVe the following as the proportional 

 quantities discharged through the principal channels of excretion. The mean 

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

 in spring^) being 91 oz., or about 5f Ibs., the average amount of Faeces (in- 

 cluding me solid matter of the bile) was 5 oz. ; the average amount of Urine 

 was 48i 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 oz. At other periods of the year, a variation was ob- 

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

 and by Cutaneous exhalation. 



648. 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 ( 84 and 645) ; and it is 

 probable, 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 ( 469). 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, 

 which are all formed from the nutritious fluid, may be regarded as resolving 

 themselves, by their ultimate decomposition, into these two excretions. More- 

 over, the Blood, during its circulation, gives up one portion of its constituents 

 in one part of the body another at a different situation, and so on. Thus, 

 the elaboration of Gelatin, which is deposited so largely in the solid tissues, 

 must occasion a considerable alteration in the blood: since, in its production 

 from Albumen, a certain residuum must be left ( 615). This residuum is 

 probably another important source of the products of Excretion. The same 

 may be remarked in regard to. the Nutrition of the Nervous System ( 643). 

 In several other instances, peculiarities of action in different parts will deprive 

 the Blood that passes through them of its due proportion of certain of its con- 

 stituents ; these are partly restored by its admixture in the Heart, with the 

 Blood that has returned from other parts ; but still a general alteration in the 

 character of the Blood is the result of its Circulation ; and for this alteration, 

 it is the province of the Excretory function to compensate. A striking illus- 

 tration may be found in the change of the colour and of the proportional 

 amount of free Oxygen and Carbonic Acid, which takes place in the Systemic 

 capillaries, and which is reversed in the passage of the Blood through the 

 Lungs ( 520). Moreover, it appears that two, at least, of the Excreting 

 organs have for their function to prevent the accumulation in the Blood, of 

 matters which have been taken in as food, but for which there is no demand 

 in the economy. Thus the Liver appears to be the peculiar channel for the 

 elimination of superfluous non-azotized matter ( 664) ; and the Kidney of 

 these azotized compounds, which cannot be worked up (so to speak) into 

 tissue ( 679). Particular sources for the respective contents of other Excre- 

 tions will be pointed out when they are considered in detail. 



649. A distinction has already been drawn ( 95) between the proper Ex- 

 cretions, the retention of which in the Blood would be positively injurious, 

 and those Secretions which are destined for particular purposes within the 

 system, and the cessation of which has no immediate influence on any but 

 the function to which they are destined. This distinction is one of great 

 importance, especially when it is considered with reference to the Chemical 

 Elements that are found in the two classes of fluids respectively. The solid 

 matter dissolved in those of the latter class, is little else than a portion of the 



* Edinburgh New Philosophical Journal, 1832, 1833. 



