By the expression, local action, is here meant that exerted on the part, or in the immediate neighbourhood of the part, to which the medicine is originally applied.

Some medicines are exclusively local in their operation. Such are those applied directly to any surface, with a view to some mechanical, physical, or chemical influence on that surface. It is probable that certain medicines which act physiologically, that is, through the vital properties of the system, are also exclusively local. Medicines incapable of being absorbed, if there be any, would belong to this category. An example of this kind, I am inclined to think, we have in castor oil.

Many medicines are, under certain circumstances, local in their action, which, under others, may enter the system, and operate on distant parts. Thus, substances capable of violently irritating or inflaming the surface to which they may be applied, when employed so as to produce this effect, are little apt to be absorbed, in consequence probably of the distension of the capillaries; whereas, if previously diluted so as not to irritate, they may find a ready entrance into the circulation.

Other medicines both act locally, and, entering the circulation, produce the same or dissimilar effects on distant parts. Thus, opium and chloroform operate as anodynes on the nerves of the part to which they are applied, and subsequently produce the same effect throughout the system by acting upon the nervous centres; while tartar emetic is irritant in its local operation, but sedative when acting through the circulation upon the heart. Medicines of this kind are often used in reference to their local effects.

In most cases of local action, the influence of the medicine extends more or less beyond the original surface of contact. Thus, the anodyne influence of chloroform, applied to the skin, often penetrates through this tissue to parts beneath it, and the same may be said of opium, camphor, aconite, and other medicines of the kind. Castor oil, applied to the inner surface of the alimentary mucous membrane, calls the muscular coat into action. An irritant substance often extends its effects both deeply into the tissues, and to a considerable distance superficially beyond the surface of contact. How is this result to be accounted for ? In some instances, probably, by the penetration of the neighbouring tissues, to a certain extent, either on the principle of the diffusion of liquids, or on that of capillary attraction, or of both. In others, the effect may be propagated by what Mr. Hunter denominates continuous sympathy; spreading along the course of the tissue affected, with a gradual diminution, until quite lost. Again, it is not impossible that the influence may be extended to neighbouring parts through common nervous centres; as when the extract of belladonna, rubbed upon the eyelids, produces dilatation of the pupil, or opium in the rectum relieves pain in the ureters; in neither of which instances can the result be fairly ascribed to the circulation, as the effect should in this case be, what it is known not to be, equally great and rapid to whatever part of the body the medicine may be applied, supposing it to be equally distant from the heart, and to afford equal facilities for absorption. Finally, the effect may be secondary, as in the instance of castor oil, which directly irritates the mucous membrane, and, as any other similar irritant would do, indirectly excites the peristaltic movement, through the agency of the physiological law which determines the latter result as a necessary consequence of the former.