CONNECTIVE (CELLULAR) TISSUE.

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considered, belong, in accordance with the classification which I have already given you, to the series of connective tissues; fibro-cartilage, fibrous or tendinous tissue, mucous tissue, bone and the teeth, must one and all be considered

But to the same category

as belonging to the same class. belongs also the whole mass of what has usually been included under the name of cellular tissue (Zellgewebe), and for which the name proposed by Johannes Müller, connective tissue (Bindegewebe) is the most appropriate; that substance, which fills up the interstices in the most different organs, sometimes in greater, sometimes in less, quantity— which renders the gliding of parts one upon the other possible, and formerly was imagined to enclose considerable spaces (cells in an inexact sense of the word), filled with a gascous vapour or with moisture.

Of this kind is the peculiar interstitial, or connective, tissue, such as we meet with in the interior of the larger muscles between the several primitive fasciculi and in a still larger quantity between the several parcels, or bundles, of primitive fasciculi. Numerous arteries, veins, and capillaries lie in it; and the arrangements for its nutrition are the most favourable that can be imagined. Notwithstanding this, however, there exists in it also, in addition to its blood-vessels, a more delicate system of nutrient channels, precisely similar to that with which we have just become acquainted; only that, wherever it is specially required, in particular parts a peculiar change takes place in the cells, the place of the simple cell-networks and fibres being gradually occupied by a more compact structure, which originates in a direct transformation of them, namely, the so-called elastic tissue.

A few months after I had made known my first observations concerning the systems of tubes existing in the connective tissues, Donders published his concerning the transformation of the cells of connective, into the

elements of elastic, tissue-a discovery, which has essentially contributed to the completion of the history of connective tissue. If this tissue, namely, be examined at points where it is liable to be much stretched, and where consequently it must be endowed with great power of resistance, the

elastic fibres will be found arranged and distributed in it in the same way that the cells and cell-tubules of connective tissue usually are, and the transformation of these latter into the former can gradually be traced with such distinctness, that there remains no doubt, that even the coarser elastic fibres directly result from a chemical change and condensation of the walls of the cells themselves. Where originally there lay a cell, provided with a delicate membrane and elongated processes, there we see the membrane gradually increasing in thickness and refracting the light more strongly, whilst the proper cell-contents continually decrease and finally disappear. The whole structure becomes in this way more homogeneous, and to a certain

Fig. 43. Elastic networks and fibres from the subcutaneous tissue of the abdomen of a woman. a, a. Large elastic bodies (cell-bodies), with numerous anastomosing processes. b, b. Dense elastic bands of fibres, on the border of larger meshes. c, c. Moderately thick fibres, spirally coiled up at the end. d, d. Finer elastic fibres, at e with more minute spiral coils. 300 diameters.

FIBRES OF ELASTIC TISSUE.

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extent sclerotic and acquires an incredible power of resisting the influence of reagents, so that it is only after longcontinued action that even the strongest caustic substances are able to destroy it, whilst it completely resists the caustic alkalies and acids in the degree of concentration usually employed in microscopical investigations. The farther this change advances, the more does the elasticity of the parts increase, and in sections we usually find these fibres not straight or elongated, but tortuous, curled up, spirally coiled, or forming little zigzags (Fig. 43, c, e). These are the elements which in virtue of their great elasticity cause retraction in those parts in which they are found in considerable quantity, as, for example, in the arteries. The fine elastic fibres, which are those that possess the greatest extensibility, are usually distinguished from the broader ones which certainly do not present themselves in tortuous forms. As far as regards their origin, however, there seems to be no difference between the two kinds; both are derived from connective-tissue cells, and their subsequent arrangement is only a reproduction of the original plan. In the place of a tissue, consisting of a basis-substance and anastomosing, reticulated cells, there afterwards arises a tissue with its basis-substance mapped out by large elastic networks with extremely compact and tough fibres.

It has not up to the present time positively been determined, whether in the course of this transformation the condensation (sclerosis) of the walls of the cells proceeds to such a pitch as entirely to obliterate their cavity, and thus completely destroy their powers of conduction, or whether a small cavity remains in their interior. In transverse sections of fine elastic fibres, it looks as if the latter were the case, and there is therefore ground for the supposition, that in the transformation of the corpuscles of connective tissue into elastic fibres, nothing more than a condensation and thickening, and at the same time a chemical metamorphosis

of the membrane, takes place, but that ultimately, however, a very small portion of the cell-cavity remains. What sort of a substance it is that constitutes the elastic parts, has not been determined, because it is not possible to accomplish their solution by any means; with a part of the products of the decomposition of this tissue we are, indeed, acquainted, but nothing further is known concerning its chemical constitution. But from this no decision 'can be arrived at with respect either to its composition, or position in a chemical point of view with regard to other tissues.

This kind of transformation prevails to an extraordinary extent in the skin, especially in the deeper layers of the corium proper, and to it is chiefly owing the extraordinary resistance of this tissue which we so gratefully acknowledge when daily testing it in the soles of our shoes. For the firmness of the individual layers of the skin depends essentially upon the greater or less quantity of elastic fibres contained in them. The most superficial part of the corium immediately beneath the rete mucosum is formed by the papillary portion (Papillarkörper), by which we are to understand not only the papillæ themselves, but also a continuous layer of coriaceous substance running along horizontally beneath them; it is under this that the coarse elastic networks begin, whilst only fine elastic fibres, and these in a fascicular form, ascend into the papillæ themselves, at the base of which they begin to form fine and close-meshed networks (Figs. 16, P, P; 83, A, e; D, c). These latter are connected inferiorly with the very thick and coarse elastic network which pervades the middle and toughest portion of the skin, the corium proper; below this comes a more coarsely meshed network within the less firm, but nevertheless very solid, undermost layer of the cutis, which passes inferiorly into the adipose or subcutaneous tissue.

In the places where such a transformation into elastic tissue has taken place, there are frequently scarcely any

ELASTIC TISSUE OF THE SKIN.

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distinct cells to be found. This is the case not merely in the skin, but also especially in certain parts of the middle coat of arteries, and particularly in the aorta. Here the network of elastic fibres attains such a preponderance, that it is only with great care that minute cellular elements can here and there be detected. In the skin, on the other hand, in addition to the elastic fibres, a somewhat greater number of small corpuscles are found, which have retained their cellular nature, though they are certainly so extremely minute that they must be specially sought for. They generally lie in the interstices of the large-meshed networks, where they either form a system with perfect anastomoses and small meshes, or else appear in the shape of more isolated, roundish bodies, in consequence of the individual cells not being very distinctly connected with one another. This is especially the case in the papillary portion of the skin, which both in its continuous layer and in the papillæ contains nucleated cells, in direct contrast with the corium proper, which at the same time is less vascular. But a far greater number of vessels was certainly needed in the former part, inasmuch as they have at the same time to furnish nutritive material for

the whole stratum of cuticle which lies above the papillæ ; nevertheless, however, there is left only a small quantity of juice at the disposition of the papillæ as such. Every

Fig. 44. Vertical section from an injected preparation of the skin. E. Epidermis. R. Rete mucosum. P. Papillæ of the skin, with their ascending and descending vessels (loops). C. Cutis. 11 diameters.