In this volume we can only hope to deal with them in brief, and must select such as appear to have the greatest bearing on the veterinarian’s everyday practice.

Always prolific of heated discussion has been one question:  ’Are the horny laminae secreted by the sensitive?’ To answer this satisfactorily, it will be best to give a short account of the mode of production of the hoof in general.

A. DEVELOPMENT OF THE HOOF.

Starting with the statement that it is epidermal in origin, we will first consider the structure of the skin, and follow that with a brief description of the structure and mode of growth of the human nail, a short study of which will greatly assist us when we come to investigate the manner of growth of the horse’s hoof.

THE SKIN is composed of two portions, the EPIDERMIS and the CORIUM.

THE EPIDERMIS is a stratified epithelium.  The superficial layers of the cells composing it are hard and horny, while the deeper layers are soft and protoplasmic.  These latter form the so-called Retae Mucosum of Malpighi.

[Illustration:  FIG. 23.—­VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER RANVIER) A, The horny layer of the epidermis; B, the rete mucosum; a, the columnar pigment-containing cells of the rete; b, the polyhedral cells; c, the stratum granulosum; d, the stratum lucidum; e, swollen horny cells; f the stratum squamosum.]

Commencing from below and proceeding upwards, we find that the lowermost cells of the rete mucosum, those that are set immediately on the corium, are columnar in shape.  In animals that have a coloured skin these cells contain pigment granules.  Directly superposed to these we find cells which in shape are polyhedral.  Above them, and forming the most superficial layer of the rete mucosum, is a series of flattened, granular-looking cells known as the stratum granulosum.

Immediately above the stratum granulosum the horny portion of the epidermis commences.  In the human skin this is formed of three distinct layers.  Undermost a layer of clear compressed cells, the stratum lucidum.  Next above it a layer of swollen cells, the nuclei of which are indistinguishable.  Finally, a surface layer of thin, horny scales, the stratum squamosum, which become detached and thrown off in the form of scurf or dandruff.  In the skin of the horse, except where it is thickest, these layers are not clearly defined.

It is the Malpighian layer of the epidermis that is most active in cell division.  As they are formed the new cells push upwards those already there, and the latter in their progress to the surface undergo a chemical change in which their protoplasm is converted into horny material.  This change, as we have already indicated, takes place above the stratum granulosum.

In addition to its constant formation of cells to replace those cast off from the surface, the active proliferation of the elements of the Malpighian layer is responsible for the development of the various appendages of the skin, the hairs with their sebaceous glands, the sweat glands, horny growths and the hoof, and, in the human subject, the nail.  These occur as thickenings and down-growths of the epithelium into the corium.