Absorption by the skin of such substances as tar, iodine, arsenic and a few others is undoubted, even though not applied in ointment or oily form. Many volatile materials are easily absorbed to a varying degree. The naturally lubricated, firm, horny layer of the epidermis, with its continuity interrupted only by the glandular openings, offers the chief obstruction to absorption by the skin. If the epidermis is removed and a soluble substance applied directly to the corium, absorption takes place quite rapidly. Rubbing and the removal of the oil from the surface with alcohol, chloroform or ether (before bringing a substance to be absorbed in contact with the surface) will facilitate absorption. In this way many medicated applications are introduced into the system and exert their effects almost as effectually as when given internally. It is probable that all substances which are absorbed by the skin pass through the epidermis (sometimes aided by friction), by way of the gland-ducts and hair- follicles, whose walls are only lined with a single layer of epithelia and thus present a comparatively slight barrier to absorption. Mercury in ointment is absorbed in this way in the form of vapor or after being dissolved by the acid secretions of the skin. Watery vapor is readily absorbed by the skin from the surrounding air, and water in contact with the surface may enter the epidermis in considerable quantity by soakage, without actual absorption. Micro-organisms may be absorbed by the skin; thus furuncles have been produced by rubbing into the sound skin cultures of staphylococci. According to Wasmuth, the different bacteria do not enter the skin by the sweat or sebaceous passages, but by way of the opening between the hair-shaft and the sheath.
SENSATION
This important function of the skin provides a means of protection and discrimination. Formerly it was thought that different sensations were caused by varying degrees and kinds of irritant applied to a single nerve ending, but now it is well known that there is a distinct disassociation of sensation. General sensation is provided for every part of the skin; the thinnest portions of the skin being most sensitive, and the thickest portions the least sensitive. Ordinarily contact becomes painful, if applied directly to the corium. The acuteness of tactile sensation depends on the distribution of the sensitive papillae of the corium. Where these are abundant, as in the skin of the end of the third finger, sense of touch exists in a high degree. Webber found by experiment, that at these parts two distinct sensations of touch could be felt, only one twenty-fourth of an inch apart. The middle of the thigh and forearm appear to be the least endowed with the sense of touch, the distance at which two points of contact can be distinguished in these regions being more than two inches apart.
The tactile sense not only makes known the size, shape and other properties of bodies, but it also differentiates the varieties of pain and temperature. Goldscheider believes there are two kinds of sensitive nerves of touch. The office of the tactile corpuscle, in the light of later investigations, appears to be to give greater mechanical protection to the nerve terminations. That the quality of touch can be educated to a surprising degree is demonstrated in the blind who, by their delicacy and expertness of touch, seem almost to supply a substitute for the loss of vision. The distribution of temperature sensation is very like that of common sensation and varies in different parts of the skin, but is not modified by the relative thickness of the skin to the same extent as general sensation.
Temperature perception was once thought to be a variety of general sensation. The experiments of Blix and Goldscheider not only tend to disprove this, but seem to show that there is a separate nerve mechanism for cold, for heat and for pressure. Experimenting independently of each other, these investigators found that the same irritant produced on some parts of the skin a sense of cold; on other parts heat; and on others, only the ordinary sense of pressure. It is well known that in some diseases attended with paralysis of ordinary sensation, sensitiveness to heat and cold may remain intact. The degree of temperature felt depends, in great measure, on the extent of surface exposed. For instance, one finger can be comfortably borne in hot water which would become painfully hot to the whole hand. The tip of the tongue, the fingers and face are most sensitive to temperature changes, one-half to one degree variation being readily appreciated by these parts.
The non-striated muscles of the arteries are well supplied with vasomotor nerves, and although these elements are not concerned in sensory function, they must be briefly mentioned. They control the constriction and dilatation of cutaneous vessels causing increased or diminished blood supply and hence affecting the temperature of a part. Many morbid conditions are directly affected by these actions which originate from the depression or stimulation of the nerve centers in the medulla or from a similar action exerted upon the peripheral nerve terminations. So little is known about the action of the so- called trophic nerves that, although there will be affections of the skin which will hereafter be referred to as trophoneuroses, it is impossible to give any detailed description of their working. For the most part the nutrition of the peripheral parts is influenced or controlled by the spinal cord.
American homeopath, he directed several hospitals in New York.
Professor of dermatology.
Served as Lieut. Colonel during the 1st World War.
See his book online: American homeopathy in the world war