OF SECRETION IN GENERAL. 615 



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

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

 Moreover, the Blood, during its circulation, gives up one portion of its con- 

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

 Thus, the elaboration of Gelatine, which is deposited so largely in the solid 

 tissues, must occasion a considerable alteration in the blood: since, in its pro- 

 duction from Albumen, a certain residuum must be left ( 141, b, c). 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 Sys- 

 tem ( 249). 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 constituents ; these are partly restored by its admixture in 

 the Heart, with the Blood that has returned from other parts ; but still a gene- 

 ral 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 illustration 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 ( 766). 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 ( 833) ; and 

 the Kidney of these a~otize.il compounds, which cannot be worked up '(so 

 to speak) into tissue ( 842). Particular sources for the respective contents 

 of other Excretions will be pointed out, when they are considered in detail. 



820. A distinction has already been drawn ( 278) between the proper 

 Excretions, 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 import- 

 ance, especially when it is considered with reference to the Chemical Elements, 

 that are found in the two classes of fluids respectively. The solid matter dis- 

 solved in those of the latter class, is little else than a portion of the constitu- 

 ents of the Blood, either pure, or but slightly altered ; thus, in the Lachrymal 

 fluid, the Saliva, the Pancreatic juice, the Serous fluid of areolar tissue and 

 of serous and synovial membranes, we find little else than Albuminous and 

 Saline matter, derived at once from the blood. The Caseine, which is the 

 most characteristic ingredient of Milk ( 854 b), is but a slightly-altered form 

 of Albumen; and some curious evidence has recently been obtained, that this 

 alteration commences in the Blood, and goes on during pregnancy as a pre- 

 paration for lactation.* On the other hand, the characteristic ingredients of the 

 Excretions are very different in character from the normal elements of the 

 Blood. They are all of them completely unorganizable; and they possess, 

 for the most part, a simple atomic constitution. Some of them also, have a 

 tendency to assume a crystalline form ; which is considered by Dr. Prout to 

 indicate their unfitness to enter into the composition of organized tissues. 

 With regard to some of the chief of these, there is sufficient evidence of their 

 existence, in small quantity, in the circulating Blood; but it is also clear, that 

 they exist there as products of decomposition, and that they are destined to 

 be separated from it as speedily as possible. If their separation be prevented, 

 they accumulate, and communicate to the circulating fluid a positively delete- 

 rious character. Of this, we have already seen a striking example in the 



* See Dr. G. Bird, in Guy's Hospital Reports, vol. v. 



