There remains now but to notice the arrangement of the horn tubules in the frog. The peculiar, indiarubber-like toughness of this organ is well known. Histological examination gives a reason for this.
FIG. 33.—HORIZONTAL SECTION OF HORN THROUGH THE JUNCTION OF THE WALL WITH THE SOLE. a, Horn tubule of the wall; b, horn tubule of the sole; c, d, horny laminæ.
FIG. 34.—SECTION OF FROG THROUGH CORIUM AND HORN. The long finger-like projections of corium into epidermis are sections of the long papillæ from which the horn-tubes of the sole grow. In the stainable portion of the epidermis are to be clearly seen light and dark streaks pointing out the alternate strata-like arrangement of cells mentioned in the text (Mettam).
The horn tubules of the frog are sinuous in their course. This is accounted for by the fact that in the horn of the frog there is a large amount of intertubular material, this having the effect of frequently turning the horn tubules from the straight. In addition to this, the intertubular material has a peculiar arrangement of the cells composing it. These are laid down in alternating striæ (1) of cells with their long axes longitudinal, and (2) of cells with their long axes horizontal. This is seen in Fig. 34, between the long papillæ of the corium, where the lines of longitudinally arranged cells in horizontal section stand out darker than the adjoining strata in which their arrangement is horizontal. The tortuous direction of the horn tubules, and the almost interlocking nature of the alternating strata of the intertubular material, together combine to give the frog its characteristic toughness and resiliency.
C. EXPANSION AND CONTRACTION OF THE HOOF.
Among other questions productive of heated argument come those relating to expansion of the horse's hoof. In the past many observers have strenuously insisted on the fact that expansion and contraction regularly occur during progression. Opposed to them have been others equally firm in the belief that neither took place. Quite within recent times this question also has been settled once and for all by the experiments of A. Lungwitz, of Dresden. His conclusions were published in an article entitled 'Changes in Form of the Hoof under the Action of the Body-weight.'[A]