These are remedies which, by irritation of the skin, are intended to counter or check deeper-lying affections. Counter-irritation is a very old method of treatment, and it still holds a prominent place in therapeutics. There are several degrees of skin irritation that may be produced, viz., rubefacient, or reddening, vesicant, or vesicle-producing, and epispastic, or blistering. Beyond this an irritant may produce death of tissue. There are a few drugs, such as mercuric chloride and croton oil, which attack the gland-mouths and produce pustules (pustulant effect), but these are not now employed as counterirritants. In therapeutics, in almost all cases, it is desirable to confine the irritation to the rubefacient degree. In this the superficial vessels dilate, the skin becomes red and warm, and there may be smarting. If the application is too strong or is allowed to remain too long, little vesicles appear, and presently, coalescing, form blisters.

Blistering is very rarely employed as a remedial measure. Until recently blistering of the gums by ammonia was a common practice of dentists; and today a fly-blister over the knee-joint in cases of large inflammatory effusions is more or less employed. However, in almost all cases not only is blistering not desirable, but it is distinctly harmful. For not only is the blister a painful lesion, requiring treatment of itself, but it effectually prevents further applications to the skin at that spot. Hence the more active agents, like mustard and heat, must be carefully watched, especially when the patient is suffering from severe pain or is somnolent or comatose. Unintentional blistering frequently results because of neglect to remove a mustard poultice before going to sleep. In brunets an area of blistering or even vesication may be followed by permanent pigmentation.

The mode of action of counterirritants has been the subject of much speculation, but the recognition in recent years of a relationship between the viscera and certain areas of the skin and body-wall through the nervous system has thrown much light upon the matter. Dana (1887) called attention to "referred pains" as being due to the distribution of the nerves, and Head (1893) and Mackenzie (1902) determined that tenderness of the superficial tissues might be a manifestation of inflammation or injury of one of the internal organs. Recent physiologic studies have shown that pain is elicited only in structures supplied by the cerebrospinal nervous system, and that viscera supplied by sympathetic nerves have no proper pain sense. The apparent pain in inflamed viscera is thus due to a reflex effect through the cerebrospinal nerves. Hence the tenderness of appendicitis is mostly localized at one point, though the actual situation of the appendix is very variable; the tenderness of cholelithasis is spread over an area much greater than that of the gall-bladder; and in pulmonary tuberculosis the superficial tissues are sometimes so tender as almost to preclude examination by percussion. Hertz (1911) concluded that pain in disease of the alimentary tract may be situated in the skin, muscles, and connective tissues. Sherrington (1909) demonstrated that on cutting certain nerves passing to the intestines and stimulating the central cut ends, the abdominal muscles contract in a definite manner. Also, it is a well-known physiologic fact that pain tends to cause contraction of the splanchnic arteries. Tice and Larson (1917) found that heat to the abdomen caused a rise in arterial pressure, but cold produced no essential change.

These findings all go to show a very close relation, through the nervous system, between the tissues of the body wall and the contained viscera, and tend to explain how irritation of a superficial area may have a decided effect upon a deep-lying or even remote viscus which is in no way in direct connection or contact with the irritated area.. In this way may be understood the expulsion of flatus by the intestines as the result of a turpentine stupe applied to the abdomen, though the intestines have no direct anatomic connection with the anterior abdominal wall; or the effect of a mustard foot-bath in pelvic congestion; or of a mustard paste on the chest in pleurisy or pneumonia. Muller demonstrated that the application of an ice-bag or a hot-water bag to the abdomen has little if any effect upon the temperature of the underlying viscera. But Leonard Hill states that the intrapleural temperature may be decidedly raised by a hot poultice on the chest wall.

As working theories, Head and Hertz adopt the segmental relation, i. e., that the spinal cord and brain are in regular segments, and that a lesion affecting a nerve from a given segment affects all the nerves whose centers are in that same segment. "Head's areas," mapped out on the skin by Head as being the areas of tenderness in the various visceral affections, have not, however, been at all constant, and Mackenzie has pointed out that in visceral lesions pain and tenderness do not appear in the whole distribution of any one segment, but in limited areas in the distribution of two or several segments. Therefore, Mackenzie suggests a regional relation rather than a segmental one. Langs troth (1915) finds that the areas over which hyperalgesia may be found in disease of each viscus are numerous. The good action of reflexes from skin stimuli may be the result of a conferred hypersensitiveness to stimuli owing to the visceral inflammation, to reflex changes in the circulation, or to other so far unknown effects.

Fig. 1.

Fig. 1. - Areas in which pain is some-times felt: (A) In cardiac affections; (B) in affections of the stomach; (C) in affec-tions of the liver, stomach, or duodenum; (D) in affections of rectum or uterus (after James Mackenzie, in "Symptoms and Their Interpretation").

Rubbing the back will sometimes distinctly affect the viscera, and Mackenzie's picture herewith suggests a reason for the success, in some instances, of the osteopathic plan of manipulating the spine and its neighborhood.