ADAPTATION TO EXTREME TEMPERATURES. 209

of adaptation on the part of the living system, to great extremes of heat; while the temperature of the blood may not be allowed, owing to the cooling influence of so much exhalation, to become more than a few degrees, and perhaps under no circumstances more than 12° higher than its natural temperature. At the same time, the nonconducting character of the tissues of the body may not be without some degree of influence, in enabling the body to resist sudden and great changes of temperature, before the increased secretion of the exhalants is able to exert its important effect in this respect. The belief in a large degree of influence in this matter being due to the non-conducting character of the skin itself, is much supported by the impunity with which the human system has been exposed to very low degrees of atmospherical temperature; in which cases, a much diminished degree of exhalation from the vessels of the surface would hardly suffice to explain the resistance of the system to the depressing and stagnating influence of an extreme degree of cold. Either owing to the skin acting as so bad a conductor of heat, or to diminished exhalation, or to the combined effect of several concurring causes, the human body has been exposed, without inconvenience, or without ultimate injury, to a temperature of 40°, or even more, below the zero of Fahrenheit's scale.

Supplied with a very large amount of nervous filaments, to endow it with sensation, the skin acts as the mind's great and watchful sentinel against

danger, and as the most ready and accurate informant on all points connected with external things; and the surface of the body becomes, consequently, a possible means of extreme physical suffering, and of material and direct injury to the sensorial functions; and, per contra, the skin affords a ready means by which the nervous system may be influenced, and either soothed, or excited, according to circumstances. Furnished with a very large supply of blood, to afford to its own substance the means of being continually renewed, to supply its secreting organs, and to keep up the energy and healthiness of the nerves, which are dependent on the due supply of blood, the skin has to be looked upon as an organ by influencing which, a very large quantity of blood may be suddenly thrown into the interior of the system, with corresponding engorgement of the viscera, and perhaps an irreparable degree of injury; or, as an organ, by determining or attracting the blood to which, the viscera and membranes of the interior may be proportionably relieved from undue pressure, or congestion, and perhaps enabled to resume the performance of the natural and necessary functions, which it is their business to discharge, and which such determination of blood to an internal organ, or such engorgement, might have interfered with or interrupted.

The skin, strictly so called, is covered by a much thinner layer, called the cuticle; a layer which may, without impropriety, be called inorganic, inasmuch as it is destitute of blood-vessels, nerves, and ab

STRUCTURE OF THE SKIN.

211

sorbents. It appears to be secreted from the outer surface of the subjacent tissue, in the form of a semifluid or viscid substance, which speedily loses the superfluous moisture, and becomes sufficiently hard to protect the sentient tissue beneath it from undue contact with external matter; while its thinness, and perfect flexibility, and adaptation to the surface, prevent it from interfering disadvantageously with sensation, or with the other functions and offices of the true skin. By friction, it becomes detached in the form of small scales; and its degree of re-production evidently depends on the degree of attrition of the surface of the body,—and it becomes very thick and hard, almost of horny character, in the soles of the feet of such poor pedestrians as have no shoes to protect the feet from the roads, and in the hands of many handicraftsmen. The cuticle is the thin membranous covering which is detached by blisters, burns, and other similar means of external injury or irritation. If the blister applied be of more severe character, or the surface of the individual be peculiarly sensitive to the influence of such an agent, a second membrane is often easily detached by means of a pair of forceps, or even of the fingers. It is sometimes advisable or necessary to effect the removal of this second tissue,-as when it is wished to promote a long-continued and purulent discharge from the vesicated surface. This second tissue is much thicker than that already spoken of; and is probably identical with a much more marked tissue, which is met with in the negro variety of the human

race. In the negro, this second tissue, called the mucous tissue, is the seat of the black colour of the skin; and in the different shades of colour or tint in the different varieties of the human family, it seems to be in this tissue that such colour resides. In minute structure, this, as its technical name, rete mucosum, implies, seems to consist of a fine and intricate network, through the meshes of which the excretion of the cuticle or epidermis is accomplished. The colouring matter or pigment of the rete mucosum, is deposited on its inner surface.

The corion, or true skin, consists of closely interlaced, minute, and dense fibres, more firmly compacted on the outer than on the inner surface. Its inner surface is marked by depressions, termed areola; but its outer surface is quite smooth, and so much more vascular, that after a successful injection of its vessels, it appears to be covered with a uniform net-work of minute vessels, infinitely subdivided, for the most part containing only colourless bloodglobules, unless under circumstances of irritation or disease. This net-work is penetrated by the papillæ, which come into contact with the cuticle, and are the seat of the sense of touch. The papillæ are

highly vascular and nervous in their structure. When the surface is chilled, the corion shrinks, whether from the diminished amount of blood contained in its vessels, or from a true contraction of its substance; whereas the papillæ either continue unchanged, or shrink less in proportion,—giving rise to what has been called goose skin.

EFFECT OF TEMPERATURE ON THE SKIN. 213

Warmth first increases the action of the vessels of the skin, and causes a more considerable efflux of the perspirable matter; the skin is sensibly reddened; and, if the heat is increased in degree, inflammation, blistering, contraction, wrinkling of the tissue, and blackening, with disorganisation, may ensue. Cold at first stimulates the skin, and reddens it, from the excited action, and greater fulness of the capillary vessels. If continued, and the cold be sufficiently intense in its degree, the vessels contract, and the skin becomes pale,—and, if the cold is increased in its intensity, loss of feeling, and eventually death of the part, ensues. This account involves the supposition, that the surface of the body is surrounded with the air.

If the body is surrounded with water, the effect is somewhat different. When the system is in a state of vigour, and the surface is unchilled, the effect of immersion in warm water is much the same as that from a moderate degree of heat applied in other ways, excepting that the greater pressure on the surface must be allowed for, as well as the effect of the damp atmosphere respired, produced by the steam from the warm water of the bath; and the respiration is accordingly more laborious in its character, and in general the skin more actively pours out its secretions. It is indeed quite true that if the parts of the surface that are immersed in the water be subjected to friction, or if the trunk and limbs be moved about a good deal under the surface of the water, a corresponding proportion of the water of the bath is