The surface of the body is of great interest and importance to the therapeutist, because it is the region of objective impressions, where influences of every kind may be brought in contact with nerves and vessels, and through them with the nervous centres, the circulation generally, and indeed the entire system. The measures applied to this part appear at first sight to be very simple, but their action is, on the contrary, 2 1-8 extremely complex, and indeed still very obscure. On this account we have taken them last in the whole range of remedies, and it will be found that they involve all the systems already discussed, especially the nervous and circulatory. As a group they are very heterogeneous, and we will select for special consideration three distinct subjects, namely (1) Counter-irritants, such as blisters; (2) Baths; and (3) Surgical Applications.

I. Physiological Relations.-The physiological relations of the surface of the body have already been studied under several distinct heads.

The nerves are connected not only with the sensorium, but with the vital centres which regulate the vessels and viscera. The cutaneous vessels have equally extensive relations. They have the usual nutritive function; they are the great refrigerating apparatus of the body; and they also serve as a great external blood-reservoir, in connection with the systemic circulation.

II. Pharmacodynamics.-When the classes of measures given at the ends of the chapters on the circulation and nervous system are compared, it is found that several of them act on both, and that their action may be different or even opposite according to the time for which, they are applied. For these and other reasons, a number of them have been collected into a special class, and called Counter-irritants.-These measures may be thus arranged, according to the degree of their action:

1. Rubefacients (rubere, to be red, and facere, to make) cause increased redness and heat of the parts. Such are Hot Water; Mustard, and its preparations; Ammonia, and its preparations; the confined vapour of Chloroform, Ether, and Alcohol; all Volatile Oils, especially Turpentine, Camphor, Menthol and Thymol; Iodine carefully applied; Emplastrum Picis; and Emplastrum Calefaciens.

2. Vesicants (vesica, a blister), Epispastics (Chapter XV Therapeutical Processes Connected With  22 upon, and Chapter XV Therapeutical Processes Connected With  23 I draw), or Blisters, produce a rubefacient effect, followed by the development of a blister. They include Cantha-rides, Mezereon, Ammonia long applied or confined, Iodine, Oil or Compound Liniment of Mustard, and Scalding Water.

3. Pustulants {pus, matter) produce a crop of pimples. They are a small group, consisting of Croton Oil, Tartar Emetic, Nitrate of Silver in strong solution, and Ipecacuanha.

Phenomena of counter-irritation.-When a counter-irritant is applied to the skin, the first effect is rubefacient and stimulant. The cutaneous vessels are dilated by a direct action on their nerves, and the local circulation becomes more free; whilst the irritation of the sensory nerves causes pain of a hot burning character. The cardiac action is accelerated, the cutaneous vessels generally reflexly contracted, the blood pressure rises, the temperature is elevated, and the breathing slowed. The highest centres are also roused by the painful impression: perception, consciousness, and the emotions are variously disturbed. Cutaneous anaesthesia follows: the nerves are depressed, pain is relieved, excepting that caused by the application itself.

Prolonged application is generally required to induce the second degree of counter-irritation-vesication. The reddened area now becomes inflamed; plasma escapes from the vessels, followed by corpuscles; the epidermis is raised, and a vesicle is formed containing a quantity of fluid. The previous anaesthesia is now replaced by considerable local pain, which, if extensive, may depress the viscera-weakening and slowing the heart, lowering the pulse, further slowing the respiration, lowering the temperature and diminishing nervous energy.

The third degree of counter-irritation, pustulation, is different in kind from vesication as well as more severe, the result being not uniform inflammation, but a crop of painful, "angry" pimples or pustules, which are very slow to heal. The remote effects are the same as before, but greater.

Theory of the action of counter-irritation.-Such are the phenomena of this method, obvious to all. But it is held by some that not only the functional activity, but the nutrition of internal parts may be affected by means of it. The doctrine of counter-irritation may be said to be, that when a part at some distance beneath the surface of the body, such as a joint, or even remote from it, such as the lungs, is in a condition of inflammation, pain, unnatural activity, or overgrowth, an alterative effect may be produced upon its nutrition, by altering the condition of an area of skin superficial to it, or even at a distance from it. A second or "counter" seat of "irritation" is set up to relieve the deeper and more vital part. Now we may conclude with respect to this theory:

1. That rubefacients and vesicants will afford relief to the circulation of parts in immediate vascular .connection with the selected area, by attracting blood and draining off plasma; to the same extent the general circulation will be depressed, and visceral congestion or inflammation will be diminished. At the same time the heart will be relieved.

2. That the irritation of the cutaneous nerves will modify in a simple reflex way, through the centres in the brain and cord, the circulation and nutrition generally, of the parts beneath; the impression which passes in being immediately reflected along the vascular or trophic nerves.

3. That possibly the irritation of the local nerves and vessels may affect the vaso-motor and trophic centres in the brain and cord, presiding over the area of skin; and that this disturbance may so influence a neighbouring trophic centre (say of a joint) as to produce through it a change in the nutrition of the tissues (such as a joint) in the neighbourhood of the area to which the irritant was applied.

4. That vesicants and pustulants may produce a flow of plasma or pus, which will relieve the blood or tissues of organised or other poisons, which are the cause of the disease. This is the old humoral view, founded on the pathology that "humours of the blood " are the origin of disease.

III. Pathological relations and Therapeutics.-The pathological conditions which we seek to influence by counter-irritants belong to various systems, which have been already discussed. The same remark holds true of the therapeutical applications of the principles just examined. All that remains to be done here is to enumerate the chief morbid conditions which may be treated by counter-irritation. These are, (1) Subacute or chronic inflammation, with or without unnatural growth, of parts in direct vascular connection with the skin; e.g. of a joint or bone. (2) Congestion or inflammation in neighbouring viscera; e.g. of the lungs. (3) Pain in deep or distant parts, such as neuralgic, cardiac, or renal pain. (4) Spasm, or other morbid activity in deep muscular structures, such as lumbago and vomiting. (5) Central nervous disturbances such as syncope and hysteria.

Baths and Allied Measures.-The principles on which the use of baths depend are in a great measure identical with those which we have already discussed, and do not require to be repeated. If the student will carefully bear in mind the relations of the vessels and nerves of the skin to the body heat, circulation generally, and nervous system, he will readily appreciate the subject of baths from the following tables, which give a list of the most common baths, together with their action and principal uses succinctly arranged.