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perverted; as in the case of a limb which has been removed, where, when the nerves that supplied the removed part are affected with pain, this is referred to the part which has been lost, it may be, years before.
The functions of the skin as a covering for the body, adding beauty and preserving the delicate structures underneath, regu. lating the intensity of sensations from without, and (by a beautiful contrivance which we shall subsequently refer to the amount of temperature within, are a further illustration of the multiplicity of ends attained by the Creator through one and the same agency; and, though last, not least, we may mention the function of excretion, or removing from the body materials no longer of use to it, and which, if retained any longer, would become actually injurious to it.
We shall now proceed to describe the beautiful structure of the skin, by which it is adapted for the discharge of these numerous and important offices. The skin is composed, as most of our readers are aware, of two layers; an outer, called the cuticle, or scarf-skin, or sometimes termed the epidermis; and an inner, called the cutis, or true skin, or sometimes the dermis. This latter rests upon a very fine interlaced or netted structure, called the areolar tissue, out of which, if we may so express it, the granules and fibres of the skin are formed.
It has been usual to describe a third layer placed between the true skin and the scarf-skin, and called the rete mucosum, or pigment layer ; but later researches have shown that there is no such distinct layer, and that the pigment cells, to which the colour of the skin in different races is due, are but a different stage in the development of the scarf-skin. This scarf-skin is never of very great thickness in any animal, but the true skin is of very variable thickness, and is that portion of the skin on which depends the thickness of the hide of the pachydermatous animals, a character so remarkable as to give name to the class to which they belong, which includes such animals as the rhinoceros, hippopotamus, elephant, horse, pig, etc. In the whale the cutis attains the thickness of about an inch, which is the greatest known in any animal.
It seems the most natural method in describing the skin to begin with the cuticle, which is at the surface, and so proceed from the better known to the less known, as in most other matters of knowledge. The cuticle, then, consists of several layers of laminated scales, the laminated form being best marked at the very surface, where the scales are constantly falling off as a kind of scurf, and are as constantly being renewed from below. These scales are formed by the flattening out of granules more or less rounded, which is the form assumed by the particles of the cuticle in its deeper layers; these granules are at first nucleated cells, and the colouring matter of the skin resides in the nuclei, and these granules it was that were formerly described as a separate layer under the name of the rete mucosum. They are very minute, being about 1-3000th of an inch in diameter at first; being renewed from below as the flattened scales are removed above, they gradually approach the surface, and as they do so they more and more lose the granular form and assume the scaly character, their diameter increasing accordingly to about 1-600th of an inch. In many animals, however, they are much larger than this, for the scales of reptiles and fish are indeed only a modified form of these epidermal scales in man; and in some of these creatures, as serpents, the epidermis, instead of being in a constant state of renewal and repair, as in man, is only removed at one particular season, when it comes off en masse, and is called the slough of the reptile. As may be supposed, the body of the animal is very tender after this process, and it goes off and hides itself for a season, until nature has repaired the loss of the old epidermis by a new one. Something similar to the gradual hardening of its new skin which then takes place we see in ourselves in the gradual hardening and thickening of a new nail, if we accidentally lose one, which we may observe to grow in thickness as well as in length. In fact, the nails are nothing more or less than modified cuticle placed in the position we find them in order to give protection and support to the ends of the fingers, and so enable the tips of the fingers, which are the tactile organs in man, duly to appreciate the nature of the bodies with which they come in contact; and it is found that the tactile sensibility of the finger is much impaired by the loss of the nail. In some of the lower animals the nails are further modified into claws, so as to become weapons of defence and offence.
Into the epidermis or cuticle no nerves or blood vessels penetrate, and it is nourished merely by the transudation of the serum of the blood through the walls of the vessels of the true skin and subcutaneous areolar tissue; and as it has no nerves it is not itself sensitive, but on the contrary, serves to blunt the too exquisite sensation of the true skin. That it has no sensibility of its own may be proved when a small portion of it is detached from the underlying surface of the true skin, as by a blister; and this is the best way of demonstrating the cuticle in a living person, as it is extremely difficult to detach any portion of it by mechanical means.
Next we come to the structure of the cutis, or true skin, which is much more highly organized, and consists of two kinds of tissue, namely, white and yellow fibres; the former being denser and more resisting, and being therefore present in greater quantity wherever resistance is most needed, as in the palm of the hand and sole of the foot; while the yellow fibres are a highly elastic tissue, owing to their minute fibrillæ being arranged in interlacing curves, and these fibres cross each other repeatedly, and branch so as to form minute lozenge. shaped interstices, which are filled up principally by the white fibres. These yellow fibres, accordingly, as might be antici. pated, exist in greater abundance where elasticity is a special requirement, as at the flexures of the joints, the lips, etc.
The uppermost surface of the cutis or true skin is strangely uneven and irregular, being elevated into a vast number of minute papillæ, which are about 1-100th of an inch in length, and 1-250th of an inch in diameter. Minute as these little papillæ are, each possesses a ramification of vascular capillaries and of nerve fibres; the latter, though not traceable to the very surface-being in fact the essential agents in the sense of touch, for that is the function of these papillæ,—they are the seat of the tactile power, and accordingly we find them developed in the greatest number and perfection where the tactile power is highest, as along the tips of the fingers and the lips in man, the lips specially in many quadrupeds, as the horse-these organs being the principal seats of the tactile power in them ; also along the membranous expansion of the wings in bats, where the sense of touch and appreciation of impact are so delicate that the animal, even though blinded, can fly between suspended threads without touching them. These papillæ are also very well developed on the trunk of the elephant, the snout of the tapir, and at the roots of the hairs of the whiskers in the feline tribe, as well as on the undersurface of the prehensile tails of some of the monkey tribe, where the sense of touch is so delicate that they can ascertain by clasping it in their tails whether a nut has a sound kernel or not, and so save themselves a useless trouble and disappointment in cracking it if unsound. These papillæ are quite distinguishable on looking at the hand, for their extremities are received into depressions on the under-surface of the scarf-skin, and when this is stripped off and examined with a low power these pits or depressions are well seen, arranged in single or double rows, which correspond with the papillæ beneath, and above with the grooves or furrows which are visible on looking at the palm of the hand and inner surface of the fingers. These furrows are caused by the scarf-skin dipping in between the rows of papillæ, and all along each furrow at very minute intervals may be seen little cross lines which indicate the separations of the individual papillæ, or rather pairs of papillæ, for they are usually arranged in pairs. The number of these papillæ is immense; a square inch of the palm of the hand will contain more than forty rows, and each row more than sixty pairs, making in all about 5,000 individual papillæ in a single square inch of skin. They are not, however, equally well developed in all parts of the body, being nearly absent on the back, where, however, the cutis is tolerably dense, for there is no relation between its thickness and the development of these papillæ ; on the tongue, for instance, the cutis is extremely thin, and yet the papillæ there are larger than in any other part of the body, and not that alone, but so thin also is the cuticle here that the individual papillæ are seen, giving that peculiar roughness to the tongue which is found to a certain degree in man, and to a very high degree in some of the lower animals, as the ox and the cat tribe.
Professor E. H. Weber instituted some delicate experiments on the sense of touch with the view of ascertaining its relative delicacy in different parts of the body, the method he adopted being to ascertain at what distance from each other two points of contact ceased to be perceived as one only, and were distinctly recognised as two. For this purpose he slightly blunted the points of a pair of compasses with sealing-wax and then applied them to different parts of the body. He then found that on the pulp or soft part of the tip of the fingers the points were perceived as two when separated only 1-36th part of an inch, while on the middle of the arm and thigh they had to be separated as much as 21 inches. He also found, as might be expected, that they were more readily perceived as two when placed across the direction of the branches of the nerves than when placed parallel to their branches. A well-known fact is that the sensations of heat and cold, which of course appertain to the sense of touch, are to a certain degree relative; that is to say, that if we place one hand in warm water and the other in cold, and then plunge both into a vessel containing water at an intermediate temperature, this will appear hot to the hand which has been in the cold water and cold to the other. Weber has also shown a very curious fact, namely, that if both hands are plunged into water of the same temperature without previous preparation it will seem warmer to the left hand than to the right. To obtain an accurate result this experiment ought to be performed with the eyes blinded, and in ignorance of the relative temperature of the water in the two vessels, so as to remove the influence of reason or imagination. Some other curious phenomena regarding the sense of touch have also been found to exist, such as that if two of the fingers be crossed and then a single small object, as a pea, be placed between them, the mind will appreciate it as two objects; or, again, that if two points, as of a pair of compasses, be applied to the skin at a fixed distance, they will feel as if more widely separated when on a very sensitive part than they will elsewhere, or if drawn along the skin from a less to a more sensitive part will seem to separate as they approach the latter; and, again, that a perfectly plane or level surface may be made to appear concave by another person drawing it over the tip of the finger of one whose eyes are covered, and pressing at first strongly, then lightly, and then strongly again, or it may be made to appear convex by reversing this order of pressure; but if the pressure is regulated by the subject of the experiment himself the delusion vanishes. Indeed, all these experiments ought to be performed on a person whose eyes are blinded, and by a second party. .
The extreme delicacy to which the sense of touch may be brought by practice often receives curious illustrations. One of the best known is the ability of the blind to read raised letters; and in one case, when the sense of touch of the pulp of the fingers had been much reduced by injury, the sufferer learned to read by applying her lips to the letters. It is said that in counting rapidly a roll of bank-notes, a clerk in the Bank of England will be able to detect a counterfeit note by the touch alone, which no examination by an ordinary individual could distinguish from a genuine note, even were he aware that it was forged.
Such are some of the wonders of the sense of touch,-a sense whose impressions are conveyed to the mind by nerves set apart for that office, these nerves being the posterior roots of the spinal nerves, and the fifth and eighth cranial nerves. The fifth cranial is, indeed, a very singular nerve; for, besides having motor fibres as well as sensitive, it has some of its sensitive fibres so modified, that in the papillæ of the tongue they become the nerves of the special sense of taste; and indeed, in some of the invertebrate animals, as the crustaceans, this nerve also receives the impressions of the special senses of sight and hearing.
We come now to the function of secretion, and the description of the beautiful and complex apparatus by which that function is carried on. When we look with a simple lens, or even with the naked eye, at the delicate grooves crossing the furrows of the hand above mentioned, we find that a small orifice exists in the centre of each of them, sometimes occupying nearly the whole of the groove. This is, in fact, the orifice of a perspiratory duct; and when the hand is warm the perspiration may be observed, even with the naked eye, to issue from it, forming minute shining dots. The glands by which the perspiration is secreted are seated at the under-surface of the true skin, each embedded in a cavity in it; and they consist, like many other glands, of a ravelled tube formed of basement membrane and epithelial scales, together with true secreting structure; the materials for secretion being