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CHAPTER XXIII.

THE CONSTITUTION, GROWTH, AND DISEASES OF THE NAILS.

DESCRIPTION of the constitution, growth, and diseases

of the nails naturally follows an account of the cosmetic care and treatment of the hands and feet, for the nail is subject to unhealthiness, not only from partaking of loss of tone with the general system, but from specific disease, from the action of powerful chemical agents, and from mechanical injuries. We mentioned, in the chapter on the cosmetic care and treatment of the hands, our own individual experience with the nails, in temporarily injuring their healthy nutrition through incautiously dabbling for several months in water highly acidulated with sulphuric acid. If a cause like this could affect nails unfavorably, it can easily be realized that many others of which the reader may at present have no knowledge would seriously injure them.

We have learned, in connection with our examination of the constitution and growth of the skin, that that portion of it called the scarf-skin is pierced up to the under side of its horny layer by microscopic papillæ that rise from the surface of the corium, the sensitive skin, the true skin. In fact, these minute papillæ with single nerves may justly be regarded, so numerous and close together are they, besides being constituted like the corium, as a large extension of the corium upward to the under surface of the horny layer of the scarf-skin. Now, the nail is only a modified form of the horny layer of the scarf-skin, and the nailbed is the mucous layer re-inforced by the corium in parallel folds, instead of the corium with, as usual, a smooth general surface from which papillæ rise. The nature of the structures and forces in play is virtually the same; it is their

arrangement and deployment that are different, and the modification resulting is the exaggeration of the horny element of the scarf-skin. That is to say, in a word, that, regarding solely the principle of the mode of generation of the scarf-skin, the principle is found to be the same in the generation of the nail, and the nail corresponds with the horny layer of the scarf-skin.

The corium, or sensitive layer of the skin, is arranged in parallel ridges running lengthwise under the nail, serving not only to receive through it the sensation of touch, but to supply to it, through the mucous layer of the scarf-skin, the elements for its increase in thickness. It follows from the fact that the nailbed lies in parallel ridges and valleys of the corium, and the other fact that this peculiarly constructed surface it is which is affording to the lower side of the nail its elements of growth, that the under surface of the nail is formed as if cast in the mold of the ridges and valleys of the underlying corium. That is exactly the case, the ridges of the corium corresponding to depressions of nail, and the valleys of the corium to ridges of nail. Examine the relatively smooth surface of the outside surface of the nail, and you will perceive upon it longitudinal parallel striations or streaks differing slightly in tint. The pinker ones represent ridges of the highly vascular corium rising into a groove of the nail-substance, and the paler ones the base of ridges of the nail-substance filling grooves of the corium. The cause of this difference of appearance in the different lines of striation is that in the former case you see the red corium through less thickness of horny matter than you do in the latter case.

The matrix, or root, of the nail is embedded in a fold of the skin. In the case of the adult thumb, it lies back of the visible part of the nail to the rear by about a tenth of an inch. Just in advance of the matrix comes the little white half-moon called the lunula. Beyond that comes the general nail-surface,

partially directed in its onward course of growth by the nailgroove, as it is called, which frames its sides with folds of skin.

Let us now examine more particularly into the nail's mode of growth in length and thickness, for, as is inferable from what has been said, it does not start out from the matrix full-grown in thickness, but grows in thickness as well as in length until it passes beyond the nail-bed. Its growth in length is determined by the capacity of the individual matrix, included within the bounds which nature has set for range of growth in nails. The nail starts forward in its growth in length, perforce of the formation of its characteristic cells by the matrix. That is to say, the generative agency is also the propelling agency, for, as the matrix forms the cells and continues to form them, the nail thus created must move on the line of least resistance, and that is obviously directly away from the matrix, along the corium-grooves and the nail-groove. The nail-bed, in generating and nourishing the lower surface of the nail, having increased its thickness and given it a formation corresponding to its own, the nail readily obeys the impulse to move forward given by the multiplication of cells at the matrix. It is, moreover, rigidly confined to this course, through the fact that the corium, lying lengthwise in ridges parallel to the bed of the nails, thereby forms parallel tracks from which the nail cannot escape.

Where we see the lunula the nail is quite thin, its tendency to growth in thickness being comparatively slight there, the whiteness of the lunula indicating the comparatively small vascular supply there, vascular supply being strictly proportioned to increase of animal tissue, whether of healthy or morbid growth. But, immediately beyond the lunula the nail evidences, through its rosy hue, the existence of rich vascular supply to that portion of it, and over that portion it is that it receives the main contribution to its thickness.

It will now be understood why the nail is so sensitive.

Although the ridges and valleys of sensitive skin into which the nail fits are shielded by the dense mass of the horny matter of the nail, they are still capable of receiving the most accurate transmission known of the character of one physical condition of matter,-smoothness. This was so well known to the Romans, that they spoke of nicety of finish to a superlative degree as we speak of a thing as being accurate to a hair, by saying that it was finished ad unguem, to the touch of the nail. The Greeks had the same perception, and used a similar expression. The Greeks, and the Romans following them, tested all exquisite finish with the thumb-nail, the final fleck in modeling in clay being removed with the thumb-nail, and the finished piece of statuary being regarded as perfect only after having been submitted to the test of passing over doubtful parts the critical and appreciative thumb-nail. We have often experimented with this test, and have found it a crucial one as to smoothness. Run the middle of the end of the thumb-nail over any surface which you regard as virtually smooth, and if there is the most minute eccentricity upon it, the jar to the end of the nail will be communicated accurately to the sensitive skin beneath the nail, and accurately registered in the brain, even to the point of definition of the size and shape of the obstruction. We have often observed carpenters, painters, and other workmen, who certainly have not all familiarized themselves with the practices of the Greek and Roman artificers, automatically examining surfaces with the thumb-nail, as if the movements were strictly instinctive.

The white marks which one sometimes observes on the nail are not primarily from a defect in the nail-substance, but from one in the corium beneath it. The corium, having received some slight injury, ceased for a brief space of time, while it was recovering, to yield through the mucous layer at that point, which has passed onward since, a normal supply of nail-substance.

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