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full glare of the sun-caused a certain amount of dispersion of light in nearly all microscopical objects, and the impression communicated to the observer was, that he saw nothing else than globules. On the other hand, however, this view corresponded with the ideas common amongst natural philosophers as to the primary origin of everything

endowed with form.

α

FIG. 12.

These globules (granules, molecules) have, curiously enough, maintained their ground, even in modern histology, and there are but few histological works which do not begin with the consideration of elementary granules. In a few instances, these views as to the globular nature of elementary parts have, even not very long ago, acquired such ascendancy, that the composition, both of the primary tissues in the embryo and also of the later ones, was based upon them. A cell was considered to be produced by the globules arranging themselves in a spherical form, so as to constitute a membrane, within which other globules remained, and formed the contents. In this way did even Baumgärtner and Arnold contend against the cellular theory.

This view has, in a certain manner, found support even in the history of development-in the so-called investmenttheory (Umhüllungstheorie)-a doctrine which for a time

FIG. 13.

occupied a very prominent position. The upholders of this theory imagined, that originally a number of elementary globules existed scattered through a fluid, but that, under certain circumstances,

Fig. 12. Diagram of the globular theory. a. Fibre composed of elementary granules (molecular granules) drawn up in a line. b. Cell with nucleus and spherically arranged granules.

Fig. 13. Diagram of the investment- (cluster-) theory. a. Separate elementary granules. 6. Heap of granules (cluster). c. Granule-cell, with membrane and nucleus.

LAW OF CONTINUOUS DEVELOPMENT.

27

they gathered together, not in the form of vesicular membranes, but so as to constitute a compact heap, a globe (mass, cluster—Klümpchen), and that this globe was the starting point of all further development, a membrane being formed outside and a nucleus inside, by the differentiation of the mass, by apposition, or intussusception.

At the present time, neither fibres, nor globules, nor elementary granules, can be looked upon as histological starting-points. As long as living elements were conceived to be produced out of parts previously destitute of shape, such as formative fluids, or matters (plastic matter, blastema, cytoblastema), any one of the above views could of course be entertained, but it is in this very particular that the revolution which the last few years have brought with them has been the most marked. Even in pathology we can now go so far as to establish, as a general principle, that no development of any kind begins de novo, and consequently as to reject the theory of equivocal [spontaneous] generation just as much in the history of the development of individual parts as we do in that of entire organisms. Just as little as we can now admit that a tænia can arise out of saburral mucus, or that out of the residue of the decomposition of animal or vegetable matter an infusorial animalcule, a fungus, or an alga, can be formed, equally little are we disposed to concede either in physiological or pathological histology, that a new cell can build itself up out of any non-cellular substance. Where a cell arises, there a cell must have previously existed (omnis cellula e cellula), just as an animal can spring only from an animal, a plant only from a plant. In this manner, although there are still a few spots in the body where absolute demonstration has not yet been afforded, the principle is nevertheless established, that in the whole series of living things, whether they be entire plants or animal organisms, or essential constituents of the same, an eternal law of continuous develop

ment prevails. There is no discontinuity of development of such a kind that a new generation can of itself give rise to a new series of developmental forms. No developed tissues can be traced back either to any large or small simple element, unless it be unto a cell. In what manner this continuous proliferation of cells (Zellenwucherung), for so we may designate the process, is carried on, we will consider hereafter; to-day, my especial object only was to deter you from assuming as the groundwork of any views you might entertain with regard to the composition of the tissues, these theories of simple fibres or simple globules (elementary fibres or elementary globules).—

If it be wished to classify the normal tissues, a very simple point of view, founded upon marked characteristics, offers itself, upon which their division into three categories may be based.

We either have tissues which consist exclusively of cells, where cell lies close to cell-in fact, cellular tissue in the modern sense of the word-or we find tissues, in which one cell is regularly separated from the other by a certain amount of intermediate matter (intercellular substance), and, therefore, a kind of uniting medium exists, which, while it visibly connects the individual elements, yet holds them separate. To this class belong the tissues which are nowa-days generally comprehended under the name of connective tissues (Gewebe der Bindesubstanz), and of which what was formerly universally called cellular tissue constitutes the chief portion. Finally, there is a third group of tissues, in which the cells have attained specific, higher forms of development, by means of which their constitution has acquired a type entirely peculiar; indeed, in part so peculiar, as to appertain exclusively to the animal economy. These are the tissues which are really characteristic of animals, although a few among them exhibit transitions to vegetable forms. To this class belong the

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 fasciæ, 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 con

stitute 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

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