Читать книгу Pharmacologia - John Ayrton Paris - Страница 30

Substances which, when applied to the human body, corrugate and condense its fibres, and at the same time, exert a tonic influence through the medium of its living principle.

Astringency in any substance may be at once recognised by the organs of taste; its power in corrugating the papillæ of the tongue, and in imparting a sensation of harshness and roughness to the palate, being too peculiar to be mistaken; this is a fortunate circumstance, for there does not exist any one chemical test by which we can invariably detect the property of astringency, since it is found to reside in many different classes of substances: thus, acids, especially the stronger mineral ones, are powerfully astringent; so are many of the metallic salts, as those of iron, zinc, copper, and lead; and some of the earths, when combined with acids, of which alum is a striking example. The vegetable kingdom, however, furnishes the greater number of astringent remedies; and chemistry has shewn that this property uniformly depends upon a peculiar proximate principle, characterized by its power of forming an insoluble compound with animal gelatine; to this principle the name of Tannin has been given. As tannin generally exists in union with gallic acid, and as the latter body is known by its property of striking an inky blackness with the salts of iron, solutions of this metal were long, but erroneously, regarded as the proper test of vegetable astringency; the fallacy of this is at once shewn by the habitudes of Catechu, one of the strongest of our astringents, but which, nevertheless, will not yield the smallest degree of blackness to the solutions of iron, because it contains only tannin, the true principle of astringency, without a trace of its usual associate the gallic acid. From the power which these substances possess of astringing, and condensing the animal solids, their medicinal properties are supposed to arise, and we may perhaps, in this instance, admit such a mechanical explanation; but astringents possess also some power over the living principle of the matter which they astringe, for they are capable of acting as permanent stimulants, of curing intermitting fever, and of obviating states of general debility. Astringents would seem to moderate the morbidly increased secretions of distant parts, and to restrain hemorrhage, by their corrugating influence upon the primæ viæ,^[148] which is extended by sympathetic action to the vascular fibre; it is not difficult for any person to conceive the possibility of such a sympathy, who has ever experienced the thrilling and singular feeling which is produced over the whole body, by the acerb taste of the sloe-juice. As however the primary operation of these bodies, by their actual contact with the animal fibre, must be much more powerful than that which can result from the mere sympathy of parts, we find that the efficacy of astringents is principally displayed in the cure of diarrhœa, or serous evacuations from the intestinal canal; their operation, in checking profuse fluor albus, gleet, and the inordinate secretions of other distant organs, is much less striking and unequivocal, and it is a question whether in many of such cases the benefit arising from their use may not depend upon their tonic powers. As the morbid excess of different evacuations may arise from various and opposite states of the living system, so may the individuals of the other classes become astringents; and we are bound to admit upon this, as we have on other occasions, the existence of absolute and relative remedies.

Narcotics, at the head of which stands opium, will frequently assume the character of astringents, by diminishing the irritability upon which increased discharges depend. In Diarrhœa, an astringent, properly so called, diminishes the flow of those acrid fluids into the intestines, by which their peristaltic motions are præternaturally increased, and it consequently represses the diarrhœa; a narcotic, under similar circumstances, might not repress the flow of the acrid matter to which I have alluded, but it would render the bowels less susceptible to its stimulus, and would therefore produce the same apparent alleviation, although by a very different mode of operation. There is yet a third species of remedy, which may operate in restraining a diarrhœa of this description; not by stopping the flow of acrid matter, nor by diminishing the irritability of the intestinal organs, as in the instances above recited, but, simply, by acting chemically upon the offending matter, so as to disarm it of its acrid qualities; such, for instance, is the nature of absorbent and testaceous medicines. In the cure of hemorrhage, if it be active, that is to say, connected with a state of strong tonic contractility of the blood-vessels, a very different remedy will be required as an astringent, than in cases of passive hemorrhage, in which the vascular fibres are in a state of relaxation or collapse. Sir Gilbert Blane has offered some valuable remarks upon this subject, with a view to settle the difference of opinion which has arisen respecting the treatment of flooding after child-birth. (Medical Logic, Edit. 2d. p. 100.)

Astringents are capable of being exclusively used as local applications, and when they are so employed for the purpose of stopping hemorrhage, they are termed Styptics.^[149] With respect to these latter agents it must be confessed, that great popular error still exists, much of which has evidently arisen from deductions drawn from the effects of such remedies upon inferior animals; thus have several substances gained the reputation of Styptics, from the result which may have followed their application to the wounded and bleeding vessels in the extremities of the horse and ass; whereas the fact is, that the blood-vessels of these animals possess an inherent power of contraction which does not exist in those of man, and to which alone the cessation of the hemorrhage, fallaciously attributed to the Styptic, is to be wholly attributed. In many cases an application may owe its styptic qualities to its power of coagulating the blood around the orifice of the wound; in this way the contact of heated metal will sometimes arrest the flow of blood from a cut surface.