The interruption of any of these excreting processes, by causing an accumulation of effete matters in the blood, occasions speedy death (see EXCRETION); and Dr. Marshall Hall was perfectly correct in affirmingt, that the functions of egestion are more immediately necessary to the maintenance of life than those of ingestion. For whilst most animals may live for a considerable time without food, and many without oxygen, there are none which are not speedily killed (unless pre viously reduced to a state of torpidity) by the complete suspension of the excretory operations.
In all the cases hitherto considered, the necessity for the secreting function arises out of the changes which are continually taking place in the system at large, and which tend to produce an injurious effect upon the cha racter of the blood. We have seen, however, that even the act of liberation of effete or superfluous matters is frequently made to answer some ulterior purpose in the economy ; and we are thus led to notice the other class of secretions, in which this ulterior purpose appears to be the principal, if not the sole, ob ject of their separation. The variety of these, however, is so great, and their uses are so different, that no general statement can be made regarding them. It must suffice to re fer to a few examples, such as will show their importance in the economy of the different animals which form them. The secretion of tears for the cleansing and lubrication of the surface of the eye ; the salivary, gastric, and pancreatic secretions for the reduction and solution of the food ; the mammary se cretion for the nutrition of the offspring; the sebaceous secretions for the lubrication of the skin ; the mucous secretions for the protection of the mucous membranes ; the poisonous secretions of certain serpents, insects, &c. ; the glutinous secretion with which the silkworm weaves its cocoon and the spider its web ; the pigmentary secretion of the cuttle-fish ; the colouring matter se creted by the mantle of many of the mollusca for imparting various hues to their shells ; the strongly, odorous secretions of many ani mals, which seem generally attractive to those of their own kind, but repulsive to others; together with many others that might be cited, are sufficient to indicate that the form ation of even a very small amount of some peculiar product may be essential to the well being of the animal which furnishes it ; by contributing to the due performance of one or more of its vital functions, or by the protec tion it affords to some important organ.
Existence of the elements of secretions in the blood. — The chemical proofs which have been recently obtained of the presence of the characteristic elements of certain secreted fluids in healthy blood, have afforded the most complete evidence of that which was previously highly probable, namely, that the office of the secreting organs is more that of selection and separation than that of' conver sion. The proof is most complete and satis factory in regard to the chief elements of the urinary secretion ; but inferential evidence scarcely less conclusive exists with regard to several other substances.
The presence of urea in the blood was first clearly shown by Prevost and Dumas*, who found that when the functions of the kidney's were destroyed, either by the extirpation of those organs, or by ligature of the renal ar teries, urea could be detected in the circula tinfr fluids after a short period. Similar re sults have been obtained by other experi menters ; and pathological observation, in cases where the normal secretion has been suspended or greatly diminished (as in the ad vanced stages of Bright's disease), has equally shown that under such circumstances the presence of urea manifests itself. in the blood when duly analysed4 An interesting case has lately been put on record by Dr. Shear man, in which the secretion of true urine being temporarily suspended, in consequence of accident (a watery fluid, containing neither urea, uric acid, nor the mates, being all that was passed for some day,$), urea was ob tained in considerable quantity from the se rum of blood drawn from the arin.t It would be difficult to explain such facts in any other way, than by supposing that urea is con stantly being generated in the system, and being received into the circulating current ; that being elitninated by the kidneys, in the state of health, as fast as it is formed, it has no time to accumulate in the blood ; but that when such elimination is checked or dimi nished, whilst its formation continues, the minute quantity originally present gradually increases, so as at last to become easily de tectible by chemical processes. However probdble such an explanation might be felt to be, it is yet satisfactory to find it confirmed by direct experiment.* Simon and Marchand some time since obtained satisfactory evi dence of the presence of urea in the healthy blood of the cow ; and Dr. Garrod has lately succeeded in obtaining urea from the serum of healthy human blood. The amount, as might be anticipated, was very small, only 1-200th of a grain of urea being procurable from 1000 grains of serum.-1 The pre-existence of uric acid in the blood might in like manner be inferred from the well-known fact of its deposition in gouty con cretions : this inference, also, has been con firmed by Dr. Garrod, who has discovered uric acid in the blood of gouty subjects. It might be not unreasonably asserted, however, that the presence of uric acid in the blood is the result of a disordered condition of the system generally ; and it is hence satisfactory to find that in this case also Dr. Garrod has succeeded in obtaining the substance itself from healthy blood. He states that the amount seems liable to considerable variation, and to have some relation to the period that has elapsed since food was last taken, being least where this was longest : thus in one in stance, where food had not been taken for twenty-four hours, 1000 grains of serum yielded only 2-1000ths of a grain of uric acid ; whilst a similar quantity of serum from the blood of other healthy subjects yielded 7-1000ths ; and a like amount of serum fi.om the blood of a man of full habit, but other wise healthy, yielded 37-1000ths of a grain of uric acid.