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]

[Footnote A: _Journal of Comparative Pathology and Therapeutics_, vol.
iv., p. 191. The whole of the matter in this article, from which we have
borrowed Figs. 35 and 36, is too long for reproduction here. It forms,
however, most instructive reading, and its careful perusal will well repay
everyone interested in this most important question (H.C.R.).]