PHYSIOLOGICAL CLASSES OF MEDICINES.

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vessels, thereby occasioning a copious evacuation by stool, withoutmuch increase of peristaltic motion. But all purgatives act in bothways, though sometimes very unequally sosome affecting the peri-staltic motion principally,—others, the secretions and exhalations. Thosethat excite watery discharges are called hydragogues; as elaterium,gamboge, and jalap. Certain purgatives do not equally affect all por-tions of the canal. Thus colocynth, gamboge, black hellebore, andaloes, principally influence the large intestine. The acrid purgativespromote the secretion of bile and of pancreatic fluid, by the irritationthey produce at the termination of the ductus choledochus.

Most if not all cathartics are local irritants, and, in some instances,the purgative operation seems to depend solely on this, as in the case ofgamboge. But several others have, in addition, a specific influence overthe alimentary canal, shewn by the fact that they purge even whenapplied to wounds or injected into the veins ; as in the case of castor oil,senna, hellebore, &c.

Several purgatives become absorbed. Thus the particles of gamboge,rhubarb, sulphate of potash, and oil of turpentine, have been detected inthe blood:—senna, rhubarb, and jalap, have communicated purgativequalities to the milk :—lastly, cassia pulp, rhubarb, senna, and gamboge,have been recognised in the urine.

Cathartics may be arranged in groups, as follow:—

a. The first group contains the mild cathartics, denominated laxativesor lenitives; viz. manna, cassia pulp, tamarinds, prunes, honey, bitartrateof potash, and the fixed oils (as castor, almond, and olive oils). Thesevery gently evacuate the contents of the intestinal canal, and usuallywithout causing any obvious irritation, or affecting the general system.Manna, however, is apt to occasion flatulence and griping. Laxativesare employed in any cases where we wish to evacuate the bowels withthe least possible irritation, as in children and pregnant women, in per-sons afflicted with inflammation of any of the abdominal or pelvic viscera,with hernia, prolapsus of the womb or rectum, piles, or stricture of therectum ; and after surgical operations about the abdomen and pelvis.

h. The second group is composed of the saline or antiphlogistic purga-tives, such as the sulphates of soda, potash, and magnesia. Theyincrease the peristaltic motion of the alimentary canal, and augment theeffusion of fluids by the exhalents of the mucous surface, thereby givingrise to watery stools. They do not appear to possess the power ofinflaming the intestinal tube, nor of heating the general system. Theyare adapted for febrile disorders, inflammatory affections, plethoric con-ditions, &c.

c. The third group includes the milder acrid purgatives , such as senna,rhubarb, and aloes. These are more active substances than any of thosementioned in the preceding groups. They are acrids and stimulants,but their local action is not sufficiently violent to cause inflammation.Senna is employed where we want an active though not very acrid orirritant purgative. Rhubarb is administered in relaxed and debilitatedconditions of the alimentary canal. Aloes is used in torpid conditions ofthe large intestines, and in affections of the head. It is objectionable inpiles and diseases of the rectum.

d. The fourth group comprehends the strong acrid or drastic purga-tives; such as jalap, scanunony, black hellebore, gamboge, croton oil,