e centre of the figure are seen the
horny laminae, with their laminellae, and the sensitive laminae. The right
portion of the figure pictures the corium.

[Illustration: FIG. 32.--HORIZONTAL SECTION OF HORN OF WALL.]

Fig. 33 is, again, a horizontal section, cut this time at the junction of
the wall with the sole. To the left are seen, again, the horn tubules of
the wall, and to the centre the horny laminae. In this position, however,
the structures interdigitating with the horny laminae are not sensitive, but
are themselves horny. As the diagram shows, they contain regularly arranged
horn tubules cut across obliquely. It is this horn which forms the 'white
line.' To the extreme right of the figure are seen the horn tubules of the
sole.

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.

[Illustration: 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 laminae.]

[Illustration: FIG. 34.--SECTION OF FROG THROUGH CORIUM AND HORN. The long
finger-like projections of corium into epidermis are sections of the long
papillae 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 striae (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 papillae 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. EX