Imagens das páginas
PDF

LECTURE II.

FEBRUARY 17, 1858.
PHYSIOLOGICAL TISSUES.

Falsity of the view that tissues aud fibres are made up of globules (elementary granules). — The investment theory (Umhullungstheorie). — Equivocal [spontaneous] generation of cells.—The law of continuous development.

General classification of the tissues.—The three categories of General Histology. —Special tissues.—Organs and systems, or apparatuses.

The Epithelial Tissues.—Squamous, cylindrical, and transitionary epithelium.—Epidermis and rete Malpighii.—Nails, and their diseases.— Crystalline lens.—Pigment.—Gland-cells.

The Connective Tissues.—The theories of Schwann, Henle, and Reichert. —My theory.—Connective tissue as intercellular substance.—Cartilage (hyaline, fibro- and reticular).—Mucous tissue.—Adipose tissue.—Anastomosis of cells; juice-conveying system of tubes or canals.

In my first lecture, gentlemen, I laid before you the general points to be noted with regard to the nature and origin of cells and their constituents. Allow me now to preface our further considerations with a review of the animal tissues in general, and this both in their physiological and pathological relations.

The most important obstacles which, until quite recently, existed in this quarter, were by no means chiefly of a pathological nature. I am convinced that pathological conditions would have been mastered with far less difficulty if it had not, until quite lately, been utterly impossible to give a simple and comprehensive sketch of the physiological

ELEMENTARY FIBRES AND GLOBULES. 25

tissues. The old views, which have in part come down to us from the last century, have exercised such a preponderating influence upon that part of histology which is, in a pathological point of view, the most important, that not even yet has unanimity been arrived at, and you will therefore be constrained, after you have inspected the preparations I shall lay before you, to come to your own conclusions as to how far that which I have to communicate to you is founded upon real observation.

If you read the 'Elementa Physiologiae' of Haller, you will find, where the elements of the body are treated of,, the most prominent position in the whole work assigned to fibres, the very characteristic expression being there made use of, that the fibre (fibra) is to the physiologist what the line is to the geometrician.

This conception was soon still further expanded, and the doctrine that fibres serve as the groundwork of nearly all the parts of the body, and that the most various tissues are reducible to fibres as their ultimate constituents, was longest maintained in the case of the very tissue in which, as it has turned out, the pathological difficulties were the greatest—in the so-called cellular tissue.

In the course of the last ten years of the last century there- arose, however, a certain degree of reaction against this fibre-theory, and in the school of natural philosophers another element soon attained to honour, though it had its origin in far more speculative views than the former, namely, the globule. Whilst some still clung to their fibres, others, as in more recent times Milne Edwards, thought fit to go so far as to suppose the fibres, in their turn, to be made up of globules ranged in lines. This view was in part attributable to optical illusions in microscopical observation. The objectionable method which prevailed during the whole of the last and a part of the present century—of making observations (with but indifferent instruments) in the 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.

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 elemenFio. 12. tary parts have, even not very long ago, ac1 quired 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 Baumgartner 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 (Umhiillungstheorie)—a doctrine which for a time p^ 13 occupied a very prominent position.'

h c The upholders of this theory ima

gined, 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 grannies.

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

[ocr errors]

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—Kliimpchen), 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 taenia 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 development 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

« AnteriorContinuar »