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EPITHELIAL TISSUES.

81

direction and become very long, whilst in the other direcBut even one of these

tions they remain very narrow.

cells, when seen in trans

verse section, will present

a

FIG. 14.
b

an hexagonal shape, and if we look down upon the free surface of cylindrical epithelium, we see in it, too, regularly polygonal forms (Fig. 14, 6).

Contrasting with these, singularly irregular forms are met with in places where the cells shoot up in an irregular manner, and accordingly they are

found with remarkable constancy on the surface of the urinary passages, in their whole extent from the calyces of the kidneys down to the urethra. In all these

parts it is very common

to meet with instances in which a cell is round at one

FIG. 15.

end, whilst at the other it terminates in a point, or where it exhibits the appearance of a somewhat thick spindle, or

Fig. 14. Columnar or cylindrical epithelium from the gall-bladder. a. Four contiguous cells seen in profile, each with a nucleus and nucleolus, their contents slightly marked with longitudinal striæ; along the free (upper) edge, a thickish border, marked with fine, radiated lines. b. Similar cells, with their free (upper, outer) surface seen obliquely, so as to show the hexagonal form of a transverse section, and their thick border. c. Cells altered by imbibition, somewhat swollen up and with the upper border split into fibrils.

Fig. 15. Transitionary epithelium from the urinary bladder. a. A large cell, with excavations along its border, into which more delicate club- and spindleshaped cells fit. b. The same; the larger cell with two nuclei. c. A larger, irregularly angular cell, with four nuclei. d. A similar cell, with two nuclei and nine depressions, as seen from above, corresponding to the excavations of the border. (Comp. Archiv für path. Anat. und Phys.,' vol. iii, plate i, fig. 8.)

form, and it is

is slightly rounded on one side and excavated on the other, or where a cell is so thrust in between others as to assume a clubbed or jagged form. But in these cases also the one cell always corresponds with the other in not any peculiarity in the cell which gives rise to its shape, but the way in which it lies, its relations to the neighbouring cells, and its having to adapt itself to the arrangement of the parts next to it. In the direction of the least resistance the cells acquire points, jaggs, and projections of. the most manifold description. As they did not well admit of classification, Henle gave them the name, which has since been adopted, of transitionary epithelium, to express their gradual transition into distinct scaly and cylindrical epithelium. Sometimes, however, this is not the case, and another name for them might just as well have been adopted.

On account of the importance of the subject, I will just add a few words with regard to the cuticle (epidermis). In this it fortunately happens that, what is not the case in many mucous membranes, many layers of cells lie one above the other, and that the young layers (the rete Malpighii [mucosum]) can easily and conveniently be separated from the older ones (the epidermis proper).

On examining a perpendicular section of the surface of the skin, we for the most part see externally a very dense stratum, of variable thickness, which at the first glance is discovered to consist of nothing but flattened cells, that, when viewed edgeways, look like lines. They might be taken for fibres, piled up one above the other, and with slight differences of level making up the whole external layer. Beneath these layers we find, differing in thickness and substance, the so-called rete Malpighii, and next to this, in a downward direction, the papillæ of the skin. If, now, we examine the boundary between the epidermis and the rete, the result we obtain by nearly

EPIDERMIS AND NAILS.

33

every method of examination is, that to the innermost layer of the epidermis, very closely and almost abruptly, there succeed cells, which at first are also flattened, but in a less degree, and within which very distinct nuclei may be distinguished. These tolerably large cells mark the transition from the oldest layers of the rete Malpighii to the youngest of the epidermis. This is the

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point from which proceeds the regeneration of the epidermis, in itself an inert mass, which is gradually removed from the surface. And here is also generally the boundary, at which pathological processes set in. The farther we advance inwards, the smaller do the cells become;

Fig. 16. Perpendicular section through the surface of the skin of a toe, treated with acetic acid. P, P. Extremities of cut papillæ, in each of which a

the last of them standing in the form of little cylinders upon the surface of the papillæ (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

vascular loop, and near it little spindle-shaped, connective-tissue 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.

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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 papillæ 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 papillæ 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 papillæ, and upon them rests the rather cylindrically shaped youngest layer of the rete Malpighii ; then follow cells continually increasing in size, until at last

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