Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/150

124 unless the tissue is investigated in a very young foetus, we can only detect cell-nuclei intermixed with fibres, or nuclei, in whose immediate proximity a small fasciculus of fibres arises on both sides. At an early stage of development the tendons have a gray appearance, not having assumed the white colour of the adult tissue. This fact is probably connected with a chemical difference existing between the young and perfectly developed fibrous tissue, as in areolar tissue. The quantity of the cytoblastema in which these cells are formed, and by which the fibres and tendons, when perfected, are probably connected together, must be extremely small, and cannot in any way be demonstrated by observation. Its existence can only be inferred by analogy with areolar tissue: it will be remembered that it was proved to be present in the foetal condition of that tissue. The quantity of cytoblastema, in comparison to the fibres present, seems to me to be the principal distinction between areolar and fibrous tissue in the adult. The fibrous tissue contains a great many more fibres within a given space than the areolar does, and they are not more minute than those of the latter tissue. There is just as great a difference, however, between fibres of areolar tissue taken from different parts of the body, as there is between the ordinary fibres of tendons and the most common form of areolar tissue, so that a very gradual transition takes place.

3. Elastic Tissue. The distinction between elastic and fibrous tissue is exhibited at a very early period. But my investigations into the history of the development of this tissue are very incomplete, and extended only so far as to render it probable that it presented no exception to the principle of development from cells. I made use of the aorta of a foetal pig and the ligamentum nuchae of a foetal sheep for the purpose. The tissue taken from these two parts was very different in its general character. In a pig’s embryo, of six inches in length, the aorta had already acquired its yellowish colour and perfect elasticity. The external coat could be easily drawn off in long pieces, almost, indeed, as a distinct tube. Having drawn off a small portion of the middle coat (which, in order to avoid any suspicion of epithelium being mixed with it, was so carefully done, that the internal surface of the vessel remained