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HISTOLOGICAL CLASSIFICATION. 29

nervous and muscular systems, the vessels and the blood. Herewith is the list of tissues concluded.

You must now proceed so consider, in what respect, in this summary of the result of histological researches, a contrast is afforded to what was formerly, chiefly in imitation of Bichat, regarded as constituting a tissue. Bichat's tissues would, for the most part, not so much represent what we now regard as the subjects of General Histology, as what we must rather designate as belonging to Special Histology. For, if we regard the tissues in the light they were formerly regarded; if we, for example, separate tendons, bones, and fasciae, from one another, we then obtain an extraordinary variety of categories (Bichat had twenty-one), but there are not quite as many simple forms of tissue to correspond to them.

In accordance with modern notions, the whole domain of anatomy should first be divided into the categories of General Histology {tissues properly so called). Special Histology, then, takes up the instances, in which a combination of tissues, sometimes very different, into a single whole {organ) takes place. Thus we speak, for example, of osseous tissue; but this tissue, the tela ossea of general histology, does not of itself, form bone, for no bone consists entirely of tela ossea, but it has necessarily superadded at least periosteum and vessels. Nay, and from this simple conception of a bone, every bone of considerable size, for example, a long bone, differs; for that is a real organ, in which we can distinguish at least four different tissues. We have in it the tela ossea properly so called, the cartilaginous layer, the stratum of connective tissue belonging to the periosteum, and the peculiar medullary tissue. These several parts again are exceedingly heterogeneous in their nature, inasmuch as, for example, vessels and nerves enter into the composition of the marrow, the periosteum, &c. All these must be taken together to constitute the entire organism of a bone. Before we come, therefore, to systems or apparatuses, properly so called, the special subject of descriptive anatomy, a long series of gradations must be traversed, and in discussions we must always begin by having a clear idea of what the question is. When bone and osseous tissue are confounded together, the extremest confusion is occasioned, and so also when it is sought to identify nervous with cerebral matter. The brain contains many things which are not of a nervous nature, and its physiological and pathological conditions cannot be comprehended if they are regarded as occurring in an aggregation of purely nervous parts, and no consideration is paid to the membranes, the interstitial substance, and the vessels, as well as the nerves.

If, now, we consider the first of the classes into which we have divided General Histology, namely, the simple cellular tissues, a little more attentively, we find that those of which we can best obtain a general idea are unquestionably the epithelial formations, such as we meet with in the epidermis and the rete Malpighii, upon the external surface of the body, and in the cylindrical and scaly epithelium of mucous and serous membranes. Their general plan is, that cell lies close to cell, so that in the most favorable specimens, as in plants, four- or six-sided cells lie in immediate apposition one to the other, and nothing at all is found between them. The same is the case in many places with the scaly or pavement-epithelium (Fig. 16). These forms are evidently in a great measure due to pressure. For all the elements of a cellular tissue to possess perfect regularity of form, it is requisite that they should all grow in a perfectly uniform manner, and simultaneously. If their development takes place under circumstances such that less resistance is offered in one direction, it then may come to pass that, as in the case of columnar or cylindrical epithelium, the cells will shoot out in this one

EPITHELIAL TISSUES.

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Fig. 15.

direction and become very long, whilst in the other directions they remain very narrow. But even one of these cells, when seen in transverse section, will present 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, *).

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 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 striae; 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, e. 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 fur Path. Anat. und Phys.,' vol. iii, plate i, fig. 8.)

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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 form, and it is 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 [mucosumj) 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 papillae of the skin. If, now, we examine the boundary between the epidermis and the rete, the result we obtain by nearly

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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

Fig. 16.

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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 papillae, in each of which a

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