Not only have the nerve-centres a great influence on the skin, but the skin is capable of exerting a great influence on the nerve-centres. This is not to be wondered at, when we bear in mind the myriads of nerve-bearing papillæ with which the skin is beset. When the soles of the feet are tickled, the legs are involuntarily moved; and when the arm-pits and sides of the chest are tickled, loud laughter is the result. These two phenomena are examples of what is known as _reflex action_, i.e., the tickling produces an effect upon the nerve-endings in the skin, and this effect travelling to the nerve-centres (the spinal cord or brain) is _reflected_ to the muscles, and produces movement of the leg or laughter. When the body is suddenly immersed in cold water, a not uncommon result is a shivering and a chattering of the teeth; and when cold water is sprinkled on the forehead or chest, deep inspiration and a catching of the breath is produced. These are examples of “reflex movements,” due to impressions made upon the nerves of the skin; and since many of the results of bathing are undoubtedly due to this kind of reflex action, it is very important to bear it constantly in mind. The connection between the nervous centres (the brain and spinal marrow) and the skin is shown also in the occurrence of what is known as goose skin, or _cutis anserina_, which is caused not only by the application of cold to the surface of the body, but even more readily by the mental states which make the “Hair of our flesh stand up.” The rationale of this phenomenon is the contraction and shortening of the little muscles which we have seen to be in intimate relationship with the hair follicles. There can be no doubt also that the pigment cells, which are scattered thinly throughout our skins, are subject to the control of the nervous centres, and it is well known that the tint of the complexion will sometimes vary with emotional states, as it certainly does with physical states. These considerations are sufficient to show that the skin plays a most important part in the animal economy, as a protective, a secreting, a vascular, and a nervous organ. An all-important point to be determined with regard to the skin is its power of _absorption_--that is, its power, if any, of allowing substances to pass through it, and so reach the interior of the body. It is well ascertained that, if the surface of the skin be broken, absorption takes place with great rapidity, and that even when the skin is not broken, it is comparatively easy to get absorption of certain matters, such as mercurial ointment or extract of belladonna, provided they be applied with a certain amount of friction. We saw that the ducts of the sweat-glands perforated the skin spirally, and the friction has the effect of opening the mouths of these little ducts, so that the greasy or sticky preparation gets lodged within them and absorbed.