Friction of the skin causes first a temporary contraction of the vessels, followed by a more or less permanent dilatation, so that the skin continues red for a length of time after the irritation has ceased. This redness is accompanied by a warm glow from the increased circulation in the skin, and friction is therefore useful as an adjunct to cold baths. Besides this, friction along the extremities in an upward direction tends to aid the flow of lymph, and thus to remove the products of waste from the muscles.

The fascia covering a muscle forms a pumping apparatus for removing waste-products from the muscles (Fig. 158). It consists of two layers, a b and ef, and between these are lymph spaces, some of which, x, are seen in transverse, and others, which appear black from the injection with which they are filled, are seen in longitudinal section. Each time the muscle contracts, it becomes thicker, presses the two layers of fascia together, and drives the lymph from the spaces onwards into the lymphatics. Each time the muscle relaxes, the layers of fascia tend to separate, and lymph from the muscle, carrying with it the waste-products, fills the spaces between the layers. The action of the muscle itself thus tends to remove the waste-products which give rise to fatigue (vide Massage, p. 131), but after over-exertion their removal may be greatly aided by gentle but firm upward friction, which will have a similar action on the fascia to the alternate compression and separation of its two layers, caused by the action of the muscle itself.

Fig. 158.   Injected lymph spaces from the fascia lata of a dog. The injected lymph spaces are black in the figure. (After Ludwig and Schvveigger Seidel.

Fig. 158. - Injected lymph-spaces from the fascia lata of a dog. The injected lymph-spaces are black in the figure. (After Ludwig and Schvveigger-Seidel.)

Gentle firm friction thus lessens or may even remove entirely the feeling of fatigue and weight in the extremities after exertion. When applied to the nape of the neck, or along the spine, it is sometimes useful in headache, in nervous irritability, and in sleeplessness.

When applied between the shoulders in persons suffering from flatulence, it appears to aid the expulsion of gas from the stomach.

The effect of friction as a counter-irritant is greatly increased by the use of stimulating liniments. These are applied by pouring a little into the hollow of the hand and then rubbing it over the surface of the body, or else by soaking a piece of flannel in the liniment and rubbing the skin with it. Linimen-tum ammoniae applied thus to the chest is useful in the bronchitis of children; and linimentum camphorae compositum, B.P., or linimentum terebinthinae may be used in a similar way for adults.

In chronic inflammation of joints, liniments may be applied in a similar way. Sometimes it may be advisable also in such cases to swathe the joint in a piece of flannel or lint, soaked in the liniment so as to procure more continuous application.

Inunction. - Metallic salts are very slightly, if at all, absorbed from the skin when applied to it in watery solution, and wiped off without being allowed to dry. But when applied in the form of ointments a considerable absorption takes place, especially if lanolin be used as a basis. Advantage is taken of this, in order to obtain the general action of mercury without its local effect on the intestinal canal. For this purpose mercurial ointment is rubbed on the skin, and especially on those parts where the epidermis is thin, as under the axillae and on the inside of the thighs.

Absorption also takes place, however, through the skin of the hands, and if the ointment is not rubbed on by the patient himself, but by another person, in whom the action of mercury is undesirable, it has been recommended that the latter should cover his hands with a piece of bladder thoroughly well oiled in order to prevent absorption.

In children, instead of applying the mercurial ointment by inunction, it is customary to smear the ointment on a piece of flannel, and to keep it applied to the abdomen of the child by means of a bandage.