Thirdly, indigestible matters of various kinds taken into the stomach, such as food of very difficult solubility, and the rind, seeds, and stones of fruits; insoluble medicines in excess; various inorganic bodies which are frequently from habit, whim, or other cause, put into the mouth, and then accidentally or purposely swallowed; and all kinds of poisons; these various substances, either by the mechanical results of their presence, causing irritation or obstruction, their essential irritant property, or dangerous influence on the system through absorption, offer strong indications for the use of cathartics. Colic, dysentery, enteritis, and obstruction of the bowels, may all arise from causes of this kind, and be beneficially treated by the evacuation of the bowels.

In cases of poisoning, the general rule is first to evacuate the stomach by an emetic, and, after this has been well cleansed, to administer cathartics, in order to expel any portion that may have passed into the bowels.

2. A second indication to be fulfilled by this class of medicines, is depletion from the blood-vessels. This they effect directly by producing secretion or exhalation from the blood, and indirectly by carrying out of the body the nutritious matter, which had been prepared in the stomach and upper bowels for absorption, and which would otherwise have been added to the mass of the blood. Hence, when properly selected with a view to meet this indication, they are extremely useful in almost all acute inflammations with a rich blood, in many cases of subacute or chronic inflammation, febrile diseases of sthenic action, and plethoric states of the system generally. it is obvious that, for this purpose, the saline cathartics are peculiarly adapted, not only by their hydragogue, but also by their sedative or refrigerant property.

3. A third therapeutic effect of cathartics is to increase the secretion of the liver. Of this I shall have occasion to treat fully when on the subject of calomel, which is, far beyond all others, adapted to meet this indication.

4. Another important indication is the promotion of absorption. This cathartics accomplish through their depletory influence. By this method of action, they often prove extremely serviceable in dropsy, in the oedema of inflammatory affections, and to a certain extent, probably, in tumefactions of various kinds.

5. Revulsion is another highly important effect of cathartics. For the principles of this mode of therapeutic action, the reader is referred to volume i. page 49. Those cathartics are to be selected for the purpose which are capable of producing irritation, however slight, of the alimentary mucous membrane. Operating on the long track of the canal, though feebly on any one part, they produce a great aggregate revulsive effect, without sensible inconvenience to the patient. They are thus useful in all kinds of inflammation which admit of depletion, except when seated in the stomach and bowels themselves. in this position, if advantageous, they must act on some other principle than that of revulsion. They are peculiarly useful in cases of sanguineous determination to the head, producing vertigo, headache, apoplexy, and hemiplegia, in febrile diseases attended with cerebral fulness, in inflammation of the brain and its meninges, and in rheumatic and gouty disease. Even in the pure neuroses they may act favourably upon the same principle, calling off nervous irritation from the points where it may have been accumulated. Neuralgia has been treated effectually by croton oil acting as a purgative; and the drastic cathartics have long enjoyed a certain celebrity in the treatment of mania. They are among the most useful remedies in chorea, and to a certain extent prove serviceable in epilepsy and tetanus. Whenever, indeed, there is morbid concentration of blood and nervous force, in parts more or less distant from the alimentary canal, and no contraindication exists, cathartics are called for upon the principle here adverted to. The choice of the particular cathartic must of course be governed by the special circumstances of each case; but the general rule may be laid down, that, the more powerfully it acts on the mucous membrane of the primae viae, the more effective it is likely to be as a revulsive agent; one caution, however, being always observed, never to carry the remedy so far as to excite inflammation of that tissue, which reacts injuriously upon the system, and, through the special sympathies of the stomach and bowels with other organs, as well as through the febrile condition produced, may counteract the very purpose for which the medicine is administered.

6. Still another principle on which cathartics act therapeutically is by eliminating offending matters from the blood. This, though probably a powerful principle of operation, is necessarily, in the present state of our knowledge, rather vaguely applied. in fevers, various cutaneous eruptions, rheumatism, gout, and cases of defective or suppressed renal and hepatic secretion, matters probably accumulate in the blood, which cathartics aid in eliminating by increasing secretion into the bowels. They are among the best remedies in congestion of the kidneys with suppressed secretion, as in ordinary nephritis and acute Bright's disease, and they are almost indispensable in the treatment of jaundice.

It very frequently happens that several indications are offered in the same disease for the use of cathartics. in fevers generally, of sthenic action, they operate advantageously by removing offending matters from the bowels, by depletion, and by revulsion, especially from the head. in bilious and yellow fevers they superadd their Cholagogue action to other beneficial influences; and, in the low or typhoid affections, and indeed in all fevers of long continuance, may be supposed to act usefully by depurating the blood. The only contraindications to their use in fevers, is a degree of debility which forbids all depletory or exhausting measures, an already existing diarrhoea, and inflammation of the alimentary canal; and even these contraindications are rather relative. than absolute.