466 OF SECKETION AND EXCRETION. 



of chemical transformation by the agency of these cells ; the characteristic 

 materials of the several secretions not being always found to pre-exist as 

 such in the blood. 



382. A distinction may be drawn as regards this point, between those 

 Excretions, the retention of whose materials in the Blood would be positively 

 injurious, and those Secretions, which are destined for particular purposes 

 within the system, and the suspension of which has no immediate influence 

 on any other functions than those for which they are respectively destined. 

 The solid matter dissolved in the fluids of the latter class, is little else than 

 a portion of the nutritive constituents of the Blood ; either so little altered 

 as still to retain its nutritive character, as is the case with the casein of Milk, 

 and with the albuminous constituent of the serous fluid of areolar tissue and 

 of serous and syuovial membranes ; or in a state of incipient retrograde 

 metamorphosis, as seems to be the case with the peculiar "ferments" of the 

 salivary, gastric, pancreatic, and intestinal secretions. On the other hand, 

 the characteristic ingredients of the Excretions are very different in charac- 

 ter from the normal elements of the blood. They are all of them completely 

 unorganizable ; and they possess, for the most part, a simple atomic consti- 

 tution. Some of them also have a tendency to assume a crystalline form ; 

 which, as Dr. Prout justly remarked, indicates 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 com- 

 municate to the circulating fluid a positively deleterious character. Of this, 

 we have already seen a striking example in the case of Asphyxia ( 323) ; 

 and the history of the other two principal Excretions, the Bile and Urine, 

 will furnish evidence to the same effect. As a general fact, then, it may be 

 affirmed, that the materials of the proper Excretions pre-exist in the Blood, 

 in a state nearly resembling that in which they are thrown off' by the secret- 

 ing organs ; and that, as their presence there is the result of the destructive 

 changes that have taken place in the system, they cannot be retained in it 

 without injury ; but that the materials of those Secretions which are destined 

 to perform some particular function within the economy, are derived from 

 the nutritive substances which are appropriated to its general purposes. 



383. Notwithstanding that, under ordinary circumstances, the several 

 parts of the Excretory apparatus are limited, each to its own special func- 

 tion, we find that there are certain complementary relations between them, 

 which makes the action of one vicarious to a certain extent with that of 

 another. Such a relation seems to exist, for instance, between the Lungs on 

 one side, and the Liver and Intestinal glandulse on the other; for, the more 

 active the respiration, the less bile is secreted ; whilst, if the respiration be 

 lowered in amount by inactivity of body and a high external temperature, a 

 larger proportion of unoxidized or imperfectly oxidized excrementitious mat- 

 ters accumulates in the blood, giving rise to that augmented production both 

 of the biliary and of the fecal excretions, which constitutes diarrhoea. 1 And 

 thus, on the other hand, when the liver is not adequately effecting the depu- 

 ration of the blood from the constituents of bile, an augmentation of the 

 respiration by active exercise in a low temperature gives most effectual 



1 Such is probably the occasion of the " bilious attacks " and " autumnal cholera," 

 so prevalent at the close of the; summer ; the subjects of these being most commonly 

 persons who have not reduced their consumption of food during the warm season, in 

 iiccordanee with the diminished demand for the production of heat within the body. 



