The details of death by thirst afford a fearful commentary on the above remarks ;— although, from reasons which will presently be mentioned, it will be obvious that even these cascs rarely afford us true examples of the strict exclusion of all entry of w ater from without the body. After a period of ago nizing thirst, the most distressing syrnptorns of w hich seem to be referred to the dry. and inflamed throat and fauces, the deficiency of water is gradually revealed by a diminution— which is almost a suppression—of the various secretions that normally contain a large pro portion of this liquid : namely, the sweat, the urine, and the fmces. Increasing muscular debility accompanies this change ; and is soon followed by delirium and coma, ending in death.
And, conversely, the benefits afforded by water seem to receive an almost paradoxical illustration from its effects in the opposite states of starvation and of fattening. Thus, as regards the latter process, animals are stated to fatten much more easily and quickly when allowed the free ingestion of this liquid. And Becquerel and Lehmann state, that when water is taken in excessive quantity, an in creased amount of urea is excreted from the system of the healthy human subject.* While the researches of Bidder and Schmidt+ show that, even after the withdrawal of all other ingesta, the copious use of water con cedes to the starving animal a longer dura tion of life ;—diminishing, not only the waste of its protein compounds, but those collateral results of the vital processes, which are ex emplified by the excretion of urea, carbonic acid, and salts.
Hence water, m hich forms about 85 per cent. of the milk, is a universal constituent of the food of animals; and varies only in the proportion which its amount bears to that of the solid ingredients mixed with it, or dis solved in it. In some of the lowest forms of animal life, its relative amount is so great, that the remainder of the food is only present in the state of a very dilute solution. In cer tain aquatic creatures of this kind, the medium around the animal seenis to form a dilute alimentary solution, which only requires an act of absorption by- the outer surface of its body. And even in the higher animals, in whom the other alimentary constituents are always taken into a stomach, or internal ca vity, part of the total quantity of water which really accompanies them into the system is often introduced by the same mode of absorption. So that, although the amount of water consumed by the organism has pro bably a definite relation to the activity of the vital processes, the amount of this liquid habitually swallowed by any animal is greatly affected by the quantity introduced in other ways : namely, by the proportion contained in its solid food, the amount formed by the combustion of hydrogen in its body, and the quantity absorbed by its skin from the vaporous or liquid water of the surrounding media. Thus the apparently dry food of many
berbivora is explained by the large amount of water, which is present as a chemical con stituent of such food, and which accompanies its few digestible parts into the system. And the small amount of drink taken by many of the Batrachian reptiles is chiefly clue to the active tegumentary ingestion last alluded to.
The quantity of water contained in the va rious kinds of food ordinarily made use of, will be referred to hereafter. But we may probably fix its average at about 75 to 80 per cent. (or about lilbs.) of the mixed fluid and solid food (about 6'5 lbs.) of the human subject.
5. The salts of the food constitute the fifth and last group of its constituents, and that of which we may be said to know less than any of the others. For, while many of the more important are easily recognized in the ashes of the various fluid and solid aliments in which they are usually introduced into the body, still we are often at a loss to know the precise state of combination in which they are originally present in the food, far more that in which they enter into com bination with the organism itself.
In the case of many salts, we can, indeed, trace the actual changes of composition which occur in the organism. Thus the salts composed of the various organic acids united with the alkalies, are converted into car bonates, prior to their dismissal from the body' in the urine. And it seems possible that even the sulphates ase occasionally decomposed in the alimentary canal ; their sulphuric acid being deoxidized into sul phuretted hydrogen. while their bases unite with tbe carbonic acid fonned in the system.
Hence, although a careful and repeated ana lysis of the salts contained in the organism and in its total excretions, might afford some clue to the qualities and quantities of the salts which ought to be introduced in the food, it would not by any means represent the details of these deniands. While it is hardly necessary to add, that no such series of examinations has ever yet been made ; and that, however carefully conducted, it might easily overlook very small quantities of im portant ingredients. Many discrepancies, however, it would probably clear up ; such as vvhy animals which in one region seem indifferent to salt, in others seck it with the greatest avidity ; — why the diet which pro duces scurvy in one person, leaves another little affected ;—and finally, why the roving population of the South American Pampas can maintain a robust health on the fresh meat of the wild cattle which range these plains, while an apparently similar diet on the flesh of tame cattle has been known to destroy English soldiers.