HORNY TISSUES were formerly regarded as extremely simple in their structure, and as being only different forms of a substance to which the term lmratin (from l:eras, a horn) was applied. Recent investigations, however, show that the parts which consist of horny tissue—as, for example, the persistent horns of the ruminants, the epidermis, the nails, claws, and hoofs, whalebone, tortoise-shell, etc.—have a somewhat com plicated, and, in sonic respects, a variable structure, although they are so far analogous to one another that they proceed from nucleated cells which are not morphologically developed like the cells of most other organs, but which, to a certain extent, dry up and are only agglutinated together by an intercellnlar substance. In a chemical point of view, they also closely resemble one another, for when compared with other tissues they all contain a large quantity of sulphur, in combination with a substance whose origin from or affinity with the proteins bodies (q.v.) is sufficiently obvious from their behavior towards certain re-agents (the caustic alkalies and the mineral and acetic acids, for example), and their percentage composition.
If a section of horn is examined in its natural state, it appears to consist of number less bundles of fine threads lying side by side. After the addition of potash solution, i these bundles are seen to unfold into little plates, which gradually expand into regular nucleated cells.
Time cellular structure of hoofs, whalebone, tortoise-shell, etc., may be exhibited in a similar. manner. It is to time histo-chemical investigations of Mulder and Donders that we are mainly indebted for our knowledge of the structure of these tissues, w'ho seem to have established that every horny tissue contains at least three different kinds of sub-:, stances—viz., 1. The substance of the cell-membranes, which is exceedingly difficult of solution in alkalies, and which forms time principal part of the tissue; 2. The cell-! contents, which dissolve more readily in alkalies; and 3. A connecting or true intercel lular substance. • ' niniti7Pri Piii Mirmcnft (iFt) These tissues have been submitted to ultimate analysis, after having been previously digested in water, alcohol, and ether. The analogy of their composition is shown in the following tabular view: These tissues differ slightly in the quantity of inorganic matter which they contain, but the difference does not vary much beyond 1 per cent.
Hair yields from 0.54 to 1.85 per cent of ash, containing, amongst other ingredients, peroxide of iron and a little silica. In feathers, the quantity of silica is very. consider able, and it is doubtless to this constituent that the shaft in a great measure owes its strength and hardness.