In the case of some of the salines, the secretion is greatly increased, while the peristaltic movement is so little affected that the secretion may lie so long in the intestine as to be reabsorbed, and the drug therefore fails to produce purgation at all. For this reason it is usual to combine such salines with simple purgatives, which will accelerate the peristalsis.

Laxatives have little action on the system beyond that which is due to the removal of waste and irritating substances from the bowels; but simple purgatives, and still more drastic purgatives, in addition to the direct action upon the bowels, exert an indirect effect upon the blood, removing from it a not inconsiderable portion of its fluid, and therefore causing a form of partial depletion.

The action of cholagogues will be more particularly considered in another paragraph (p. 404).

The action of purgatives generally, and especially of saline cathartics, has been a subject of very great dispute, and it is a matter of extreme difficulty to determine exactly. The question seems to be, however, settled by the masterly researches of Dr. Matthew Hay, and I cannot, I think, do better than give his conclusions in his own words.

1. A saline purgative always excites more or less secretion from the alimentary canal, depending on the amount of the salt and the strength of its solution, and varying with the nature of the salt.

2. The excito-secretory action of the salt is probably due to the bitterness as well as to the irritant and specific properties of the salt, and not to osmosis.

3. The low diffusibility of the salt impedes the absorption of the secreted fluid.

4. Between stimulated secretion on the one hand, and impeded absorption on the other, there is an accumulation of fluid in the canal.

5. The accumulated fluid, partly from ordinary dynamical laws, partly from a gentle stimulation of the peristaltic movements excited by distension, reaches the rectum and produces purgation.

6. Purgation will not ensue if water be withheld from the diet for one or two days previous to the administration of the salt in a concentrated form.

7. The absence of purgation is not due to the want of water in the alimentary canal, but to its deficiency in the blood.

8. Under ordinary conditions, with an unrestricted supply of water, the maximal amount of fluid accumulated within the canal corresponds very nearly to the quantity of water required to form a 5 or 6 per cent. solution of the amount of salt administered.

9. If, therefore, a solution of this strength be given, it does not increase in bulk.

10. If a solution of greater strength be administered, it rapidly increases in volume until the maximum is attained. This it accomplishes in the case of a 20 per cent. solution in from one hour to one hour and a half.

11. After the maximum has been reached, the fluid begins gradually and slowly to diminish in quantity.

12. Caeteris paribus, the weaker, or in other words, the more voluminous the solution of the salt administered is, the more quickly is the maximum within the canal reached; and accordingly purgation follows with greater rapidity.

13. Unless the solution of the salt is more concentrated than 10 per cent. it excites little or no secretion in the stomach.

14. The salt is absorbed with extreme slowness by the stomach of the cat.

15. The salt excites an active secretion in the intestines, and probably for the most part in the small intestine, all portions of this viscus being capable of yielding the secretion in almost equal quantities.

16. The bile and pancreatic juice participate but very little in the secretion.

17. The secretion is probably a true succus entericus, resembling the secretion obtained by Moreau after division of the mesenteric nerves.

18. The secretion is promoted by local irritation of the intestine, as by ligatures, but only in the immediate vicinity of the irritation.

19. Absorption by the intestine generally is reflexly stimulated by such irritation (the effect of numerous ligatures applied at points remote from the seat of the injected salt being to diminish the amount of purgative fluid by accelerated absorption).

20. If the salt solution be injected directly into the small intestine, the stronger within certain limits the solution is, the greater will be the accumulation of fluid within the intestine.

21. This difference is not observed when the salt is administered per os, as the strong solution becomes diluted in the stomach and duodenum before passing into the intestine generally.

22. The difference is due to the local action of the salt on the mucous membrane, and probably more to an impeded absorption than to a stimulated secretion.

23. When the salt is administered in the usual manner, it appears, in the case of the sulphate of magnesium and sulphate of sodium, to become split up in the small intestine, the acid being more rapidly absorbed than the base.

24. A portion of the absorbed acid shortly afterwards returns to the intestines.

25. After the maximum of excretion of the acid has been reached, the salt begins very slowly and gradually to disappear by absorption, which is checked only by the occurrence of purgation.

26. During the alternations of absorption and secretion of the acid, it is the salt left within the intestine which excites secretion, the absorbed and excreted acid exerting no such action whilst in the blood, or during the process of its excretion, as Headland believed.

27. The salt does not purge when injected into the blood, and excites no intestinal secretion.

28. Nor does it purge when injected subcutaneously, unless in virtue of its causing local irritation of the abdominal subcutaneous tissue, which acts reflexly on the intestines, dilating their blood-vessels, and perhaps stimulating their muscular movements.

29. The sulphate of sodium exhibits no poisonous action when injected into the circulation.

30. The sulphate of magnesium is, on the other hand, powerfully toxic when so injected, paralysing first the respiration and afterwards the heart, and abolishing sensation or paralysing the sensory-motor reflex centres.

31. Both salts, when administered in the usual manner, produce a gradual but well-marked increase in the tension of the pulse.

32. According as the salt-solution within the intestine increases in amount there occurs a corresponding diminution of the fluids of the blood.

33. The blood recoups itself in a short time by absorbing from the tissues a nearly equal quantity of their fluids.

34. The salt, after some hours, causes diuresis, and with it a second concentration of the blood, which continues so long as the diuresis is active.

35. As the intestinal secretion excited by the salt contains a very small proportion of organic matter as compared with the inorganic matter, the purgative removes more of the latter than the former from the blood. In certain cases a large quantity of the salts of the blood is thus evacuated.

36. The amount of the normal constituents of the urine is not affected by the salt.

37. After the administration of sulphate of magnesium much more of the acid than of the base is excreted in the urine.

38. The salt has no specific action in lowering the internal temperature of the body, or has it only to a very small extent.

39. It reduces, however, the absolute amount of heat in the body.

Uses. - Purgatives are used, firstly, to remove from the intestinal tube faecal matters. They thus not only prevent the accumulation of such matters, but remove the irritation which their presence produces, and which may evidence itself in disturbances of other organs, producing, for example, headache and malaise. These disagreeable symptoms produced by constipation are perhaps partly due to the irritation of the intestinal nerves producing reflex disturbance of the circulation, but it is probable also that they may be due in part to the toxic action of poisonous gases, liquids, or solids, generated in the intestine by imperfect digestion or decomposition of the food. For such purposes as this we may employ, as we find them necessary, laxatives or simple purgatives.

The second use of purgatives is to remove liquid from the body in cases of dropsy, due either to heart or kidney disease. For such purposes we use saline hydragogue cathartics.

From his researches on the action of saline cathartics Dr. Hay had found that if a salt be given in a concentrated solution when the alimentary canal contains little or no fluid, it produces an almost immediate and very decided concentration of the blood by the removal of a large quantity of its water in the form of intestinal secretion. But if the salt be given in sufficient water, or if the alimentary canal contain sufficient fluid at the time of administration, no such concentration occurs. The concentration reaches its maximum in half an hour, but does not last more than half an hour or an hour, when it begins to decline, and continues to do so until it reaches the normal at the end of about four hours. This return of the blood to its normal concentration is not due to re-absorption from the intestine, but to the absorption of lymph and fluids from the tissues. Some hours after the administration, either of a concentrated or dilute saline solution, the blood undergoes another concentration, less than the first but continuing longer. Saline cathartics, as often used in dilute saline solution, owe their use in dropsy, to a great extent, to their diuretic action. When given in concentrated solution under proper conditions, the benefit they produce by purgation is exceedingly great. These conditions are that the alimentary canal should be freed from food and especially from liquids by previous abstinence for some hours, and that the salt should be given along with the smallest possible quantity of water. Sulphate of magnesium being soluble in less than its own weight of water is most suitable. Alkaline tartrates, and Rochelle salt may also be useful; sulphate of sodium is more insoluble, and therefore less suitable; phosphate of sodium and sulphate of potassium are too insoluble to be of any service.1

The third use is to lower the temperature in fever, and for this we chiefly use salines. The modus operandi here is not yet well understood, as they have no such action in health (p. 394).

The fourth use is to lower the blood-pressure, and thus to prevent the rupture of a blood-vessel, and consequent apoplexy, or to prevent further extravasation in a case where the vessel has already burst.

The regular use of aperients is especially necessary in gouty persons with contracted kidney and high blood-pressure. How far their utility is to be ascribed to their direct effect in lowering the blood-pressure, and how far to the removal of waste products which might raise the pressure it is impossible at present to say. The utility of purgatives after apoplexy has occurred may be doubtful, and though usually administered, they probably do no good. But, even if they do no good, they do no harm. A drop of croton-oil or a few grains of calomel on the tongue is the usual form of administration.

A fifth use is to prevent straining at stool where violent efforts are dangerous, as in aneurism, hernia, etc.