From the matter here given us it is easy to understand how, in a macerated foot, the appearance is given of interlocking of the sensitive and horny laminae.  We see that the horny laminae are ingrowths of the rete Malpighii, ploughing into and excavating the corium into the shape of leaves—­the sensitive laminae.  Putrefactive changes simply break into two separate portions what originally was one whole, by destroying the cells along its weakest part.  This part is the line of soft protoplasmic cells of the rete Malpighii.  Thus the more resistant parts (the horn on the one hand, and the corium covering the foot on the other) are easily torn asunder.

As a result of the evidence we have quoted, we are able to answer our original question in the affirmative.  Seeing that the horny and the sensitive laminae are both portions of the same thing—­namely, a modified skin, in which the epidermis is represented by the horny laminae, and the corium by the sensitive—­it is clear to see that the cells covering the inspreading horny laminae are dependent for their growth and reproduction upon the cells with which they are in immediate contact—­namely, those of the sensitive laminae—­and that therefore the sensitive laminae are responsible for the growth of the horny.

B. CHEMICAL PROPERTIES AND HISTOLOGY OF HORN.

Horn is a solid, tenacious, fibrous material, and its density in the hoof varies in different situations.  It is softened by alkalies, such as caustic potash or soda and ammonia, the parts first attacked being the commissures, then the frog, and afterwards the sole and wall.  Strong acids, such as sulphuric acid and nitric acid, also dissolve it.

The chemical composition of the hoof shows it to be a modification of albumin, its analysis yielding water, a large percentage of animal matter, and materials soluble and insoluble in water.  The proportions of these, as existing in the various parts of the hoof, have been given by Professor Clement as follows: 

Wall.     Sole.    Frog. 
Water                      16.12     36.0    42.0
Fatty matter                0.95     0.25    0.50
Matters soluble in water    1.04     1.50    1.50
Insoluble salts             0.26     0.25    0.22
Animal matter              81.63     62.0   55.78

Horn appears to be identical with epidermis, hair, wool, feathers, and whalebone, in yielding ‘keratin,’ a substance intermediate between albumin and gelatine, and containing from 60 to 80 per cent. of sulphur.

That horn is combustible everyone who has watched the fitting of a hot shoe knows.  That it is a bad conductor of heat, the absence of bad after-effects on the foot testifies.

[Illustration:  FIG. 31.—­PERPENDICULAR SECTION OF HORN OF WALL.]

In a previous page we have described the manner of growth of the horn tubules, and noted the direction they took in the wall; also, we have noticed the existence between them of an intertubular horn or cement.