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the last of them standing in the form of little cylinders upon the surface of the papillae (Fig. 16, r, r).

On the whole, the relations of the individual parts throughout the whole surface of the skin are everywhere the same, however manifold the peculiarities of detail may be, which the individual layers offer in respect to thickness, position, firmness, and connection. A section of a nail, for example, which in its external appearance certainly widely differs from ordinary epidermis, presents, nevertheless, on the whole, the same conformation, and has only one essentially distinctive feature, that, namely, in it two different epidermoidal structures are thrust, the one over the other, and thus a complication arises, which, if not duly attended to, may lead to the assumption of certain specific differences between it and other parts of the epidermis, whilst it really consists only in a peculiar change in the position of certain layers of the epidermis with regard to one another. The extremely dense and hard scales, which constitute the uppermost part, the so-called body of the nail (Nagelblatt), may, by different methods, be restored to forms in which they present the ordinary appearances of cells; and this is best seen after treatment with an alkali, when every scale swells up into a large, broadly oval, cell.

In the uppermost layers of the epidermis the cells become everywhere flatter, and towards the external surface no more nuclei at all can be discovered in them. Still there is no original difference between the epidermis and the NAILS. 35

vascular loop, and near it little spindle-shaped, connective-tissae corpuscles, displaying at the base a reticulated arrangement, may be observed; to the left, a bulging out of the papilla, corresponding to a tactile corpuscle, no longer visible, and situated at a deeper level. R, R. The rete Malpighii; immediately around the papilla a very dense layer of small, cylindrical cells (r, r); more externally, polygonal cells, gradually increasing in size. E. Epidermis, consisting of flat and more closely packed layers of cells. S, S. Duct of a sweat gland passing through. 300 diameters.

rete Malpighii; the latter is only the matrix of the epidermis, or indeed its youngest layer, inasmuch as from it there is a constant apposition of new parts taking place, which gradually become flatter and flatter, and move upwards as fast as the scales on the outside disappear through friction of the surface, washing, or rubbing. But between the lowest layer of the rete and the surface of the cutis vera there are no intervening layers; there is no amorphous fluid or blastema to be found there in which the cells could be generated, but they lie in direct contact with the papillae of connective tissue of the cutis. There is therefore nowhere any space here, as there was thought to be even a short time ago, into which fluid transudes from- the papillae and the vessels contained therein, in order that new cells may arise and develop themselves out of it. Of such a fluid there is absolutely nothing discernible, but throughout the whole series of the layers of cells of the rete and epidermis the same relations exist that we are familiar with in the bark of a tree. The cortical layer of a potato (Fig. 7) exhibits in a similar manner, externally, corky, epidermoidal cells, and underneath, as in the rete Malpighii, a layer of nucleated cells, the cambium, constituting the matrix for the subsequent growth of the cortex. Very much the same is the case with the nails. On examining the section of a nail, made transversely to the long axis of the finger, we see virtually the same structure as in ordinary skin, only every single indentation of the inferior surface does not correspond to a conical prolongation of the cutis, or papilla, but to a ridge which runs along the entire length of the bed of the nail, and may be compared with the ridges which are to be seen upon the palmar surface of the fingers. Upon these ridges of the bed of the nail are dwarfish, stunted papillae, and upon them rests the rather cylindrically shaped youngest layer of the rete Malpighii; then follow cells continually increasing in size, until at last the really hard substance comes, which corresponds to the epidermis.

Nevertheless—to discuss the subject at once, seeing that I shall not again have occasion to mention it—the structure of the nails has been difficult to make out, because they were conceived to be a simple formation. Nearly all the discussions, therefore, which have taken place, have turned upon the question where the matrix of the nail was, and whether the growth of the latter took place from the whole surface or from the little fold into which it is received behind. If we consider the nail with respect to its proper firm substance, its compact body (Nagelblatt), this only grows from behind, and is pushed forward over the surface of the so-called bed of the nail (Nagelbett), but this in its turn also produces a definite quantity of cellular elements, which are to be regarded as the equivalents of an epidermic layer. On making a section through the middle of a nail, we come, most externally, to the layer of nail which has grown from behind, next to the substance which has been secreted by the bed of the nail, then to the rete Malpighii, and lastly to the ridges upon which the nail rests.

Thus the nail lies in a certain measure loose, and can easily move forwards, pushing itself over a moveable substratum, whilst it is kept in place by the ridges with which its bed is beset. When a section is made transversely through a nail, we see, as already mentioned, essentially the same appearance presented as that offered by the skin, only that a long ridge corresponds to every single papilla seen in ordinary sections of the skin; the undermost part of the nail has slight indentations corresponding to these ridges, so that, while gliding along over them, it can execute lateral movements only within certain limits. In this manner, the body of the nail which grows from behind moves forward over a cmlrion of loose epidermic substance (Fig. 17, a) in grooves which are provided by the ridges and

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Fig. 17.

furrows of the bed of the nail. The uppermost part of the nail, if examined when fresh, is composed of so dense a substance that it is scarcely possible to distinguish individual cells in it without applying reagents, and at many points an appearance is presented like that which we see in cartilage. But by treating it with potash, we can convince ourselves that this substance is composed of nothing but epidermiscells. From this mode of development you will see how easily intelligible distinctions may be drawn between the different diseases of the nails.

There are diseases of the bed of the nail which do not affect the growth of its body, but may give rise to changes in its position. When there is a very abundant development of cells in the bed of the nail, the body may be pushed upwards (Fig. 17, b); nay, it sometimes happens that the nail, instead of growing horizontally, shoots perpendicularly upwards, the space underneath being filled with a thick accumulation of the loose cushiony substance (Polstermasse) (Fig. 17, c). Thus suppuration may take place in the bed of the nail without the development of its body being thereby impeded. The most singular changes occur in small pox. When a pock forms upon the

Fig. 17. Diagrammatic representation of a longitudinal section of a nail. a. The normal condition; a gently curved, horizontal nail, implanted in its fold, and separated from its bed by a thin cushion, b. A more markedly curved and somewhat thicker nail, with great thickening of the cushion, and much increased curvation of the bed, the fold being shorter and wider. e. Onychogryphosis; the nail, short and thick, reared up at a considerable angle, the fold short and wide, the bed furrowed on its surface, the cushion very thick and composed of layers of loose cells, piled up one above the other.

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bed of the nail, there is nothing to be seen but a yellowish, somewhat uneven, spot; but if, on the other hand, it is developed upon the fold, then its traces are left in the shape of a circularly depressed, and, as it were, excavated spot in the body of the nail as it gradually advances, a proof of a loss of substance precisely similar to that which takes place in the epidermis.

I will not to-day, gentlemen, enter more particularly into the special history of the formation of epidermis and epithelium, although it is of great importance for the right comprehension of many pathological processes, but content myself with calling your attention to the fact, that, under particular circumstances epithelial cells may undergo a series of transformations, through which they become extremely unlike what they originally were, and gradually assume appearances which render it impossible for those who are unacquainted with the history of their development to realize their original epidermic nature. The greatest abnormity of the kind is met with in the crystalline lens of the eye, which is originally a mere accumulation of epidermis. It has its origin, as is well known, in a saccular involution of the external skin. At first its con"nection with the external parts continues to be maintained by means of a delicate membrane, the membrana capsulopupillaris; afterwards this atrophies and leaves the lens isolated in the interior of the eye. The fibres of the lens are therefore, as C. Vogt has shown, nothing more than epidermoidal cells which have been developed in a peculiar manner, and their regeneration, after the extraction of a cataract for example, is only possible as long as there still remains epithelium in the capsule to undertake the new formation, and to represent, as it were, a thin layer of rete Malpighii. This reproduces the lens in the same way that the ordinary rete Malpighii of the external surface does the cuticle. Amongst the other changes of epithelial struc

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