Those who wish to give this subject further study will find an excellent series of articles by Fleming in the Veterinarian for 1871.  We shall content ourselves here with introducing one or two diagrams and photo-micrographs, and dealing with the histology very briefly.

Under the microscope the longitudinal striation of the wall is found to be due to the direction taken by the horn tubules.

Fig. 31 is a magnified perpendicular section of the wall.  In it the parallel dark striae are the horn tubules in longitudinal section.  The lighter striae represent the intertubular material.

Fig. 32 gives us the wall in horizontal section.  To the left of this picture we find the horn tubules cut across, and standing out as so many concentrically ringed circles.  In the 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.