_The Extremities_ of the wall are formed by the abruptly reflected portions of the wall at the heels. Termed by some the 'Inflexural Nodes,' they are better known to us as the '_Points of the Heels_.'

2. THE SOLE.--The sole is a thick plate of horn which, in conjunction with the bars and the frog, forms the floor of the foot. In shape it is irregularly crescentic, its posterior portion, that between the horns of the crescent, being deeply indented in a V-shaped manner to receive the frog. Its upper surface is convex, its lower concave. It may be recognised as possessing two faces and two borders.

_The Superior or Internal Face_ is adapted to the sole of the os pedis. Its highest point, therefore, is at the point of its V-shaped indentation. From this point it slopes in every direction downwards and outwards until near the circ.u.mference. Here it curves up to form a kind of a groove in which is lodged the inferior edge of the os pedis. In the centre of its anterior portion--that is to say, at the toe--will be seen a small inverted V-shaped ridge, which is a direct continuation of the same shaped prominence before mentioned on the internal face of the wall. This Fleming has termed the toe-stay, from a notion that it serves to maintain the position of the os pedis. The whole of the superior face of the sole is covered with numerous fine punctures which receive the papillae of the sensitive sole.

_The Inferior Face_ is more or less concave according to circ.u.mstances, its deepest part being at the point of the frog. Sloping from this point to its circ.u.mference, it becomes suddenly flat just before joining the wall. Its horn in appearance is flaky.

[Ill.u.s.tration: FIG. 21.--INFERIOR ASPECT OF HOOF. _a_ The inferior face of h.o.r.n.y sole; _b_, inferior border of the wall; _c_, body or cushion of the frog; _d_, median lacuna of the frog; _e_, lateral lacuna of the frog; _f_, the bar; _g_, the quarter; _h_, the point of the frog; _i_ the heel.]

_The External Border_ or Circ.u.mference is intimately dovetailed with the h.o.r.n.y laminae of the wall. At its circ.u.mference the sole, if unpared, is ordinarily as thick as the wall. This thickness is maintained for a short distance towards its centre, after which it becomes gradually more thin.

_The Internal Border_ has the shape of an elongated V with the apex pointing forwards. It is much thinner than the external border, and, like it, is dovetailed into the h.o.r.n.y laminae of the inflections of the wall--namely, the bars. In front of the termination of the bars it is dovetailed into the sides and point of the frog. Where unworn by contact with the ground, the horn of the sole is shed by a process of exfoliation.

3. THE FROG.--Triangular or pyramidal in shape, the frog bears a close resemblance to the form of the plantar cushion, upon the lower surface of which body it is moulded. It offers for consideration two faces, two sides, a base, and a point or summit.

[Ill.u.s.tration: FIG. 22.--HOOF WITH THE SENSITIVE STRUCTURES REMOVED. 1, Superior face of h.o.r.n.y frog; 2, the frog-stay; 3, the lateral ridges of the frog's superior surface; 4, the h.o.r.n.y laminae at the inflections of the wall.]

_The Superior Face_ is an exact cast of the lower surface of the plantar cushion. It shows in the centre, therefore, a triangular depression, with the base of the triangle directed backwards. Posteriorly, the depression is continued as two lateral channels divided by a median ridge. The median ridge widens out as it pa.s.ses backwards, forming the larger part of the posterior portion of the frog. This median ridge fits into the cleft of the plantar cushion. It serves to prevent displacement of the sensitive from the h.o.r.n.y frog, and has been rather aptly termed the '_Frog-stay_.'

_The Inferior Surface_ is an exact reverse of the superior. The triangular depression of the superior surface is represented in the inferior surface by a triangular projection, and the ridge-like frog-stay of the upper surface is represented below by a median cleft, the _Median Lacuna_ of the frog. The triangular projection in front of the median lacuna is the body or cushion of the frog. It is continued backwards as two ridge-like branches, which, at the points of the heels, form acute angles with the bars. On the outer side of each lateral ridge is a fissure. These are known as the Lateral Lacunae.

_The Sides_ of the frog are flat and slightly oblique. They are closely united to the bars and to the triangular indentation in the posterior border of the sole.

_The Base_ of the frog is formed by the extremities of its branches, which, becoming wider and more convex as they pa.s.s backwards, form two rounded, flexible, and elastic ma.s.ses separated from each other by the median lacuna. These const.i.tute the 'glomes' of the frog. They are continuous with the periople.

_The Point of the Frog_ is situated, wedge-like, within the triangular notch in the posterior border of the sole.

4. THE PERIOPLE, OR CORONARY FROG BAND.--This is a continuation of the substance of the frog around the extreme upper surface of the hoof. It is widest at the heels over the bulbs or glomes of the frog, and gradually narrows as it reaches the front of the hoof. It is, in reality, a thin pellicle of semi-transparent horn secreted by the cells of the perioplic ring. When left untouched by the farrier's rasp it serves the purpose, by acting as a natural varnish, of protecting the horn of the wall from the effects of undue heat or moisture.

CHAPTER III

GENERAL PHYSIOLOGICAL AND ANATOMICAL OBSERVATIONS

The matter embraced by the heading of this chapter will offer for discussion many subjects of great interest to the veterinary surgeon.

Around some of them debate has for many years waxed more than keen. Of the points in dispute, some of them may be regarded as satisfactorily settled, while others offer still further room for investigation.

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 h.o.r.n.y 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 a.s.sist 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 h.o.r.n.y, while the deeper layers are soft and protoplasmic. These latter form the so-called _Retae Mucosum_ of Malpighi.

[Ill.u.s.tration: FIG. 23.--VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER RANVIER) _A_, The h.o.r.n.y 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 h.o.r.n.y 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 h.o.r.n.y 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, h.o.r.n.y 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 h.o.r.n.y 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, h.o.r.n.y growths and the hoof, and, in the human subject, the nail.

These occur as thickenings and down-growths of the epithelium into the corium.

The epidermis is devoid of bloodvessels, but is provided with fine nerve fibrils which ramify between the cells of the rete mucosum.

THE CORIUM is composed of dense connective tissue, the superficial layer of which bears minute papillae. These project into the epidermis, which is moulded on them. For the most part the papillae contain looped capillary vessels, rendering the superficial layer of the corium extremely vascular.

Why this must be a moment's reflection will show. The epidermis, as we have already said, is devoid of bloodvessels. It therefore depends entirely for its nourishment upon the indirect supply it receives from the vessels of the corium. The need for extreme vascularity of the corium is further explained when we call to mind the constant proliferation and casting off of the cells of the epidermis, the growth of the hairs, the production of the horn of the hoof, and the work performed by the numerous sweat and other glands.

Others of the papillae contain nerves, ending here in tactile corpuscles, or continuing, as we have mentioned before, to ramify as fine fibrils in the rete mucosum of the epidermis.

THE HAIRS are growths of the epidermis extending downwards into the deeper part of the corium. Each is developed in a small pit, the _Hair Follicle_, from the bottom of which it grows, the part lying within the follicle being known as the _Root_. It is important to note their structure, as it will be seen later that they bear an extremely close relation to the horn of the hoof.

Under a high power of the microscope, and in optical section, the central portion of a hair is tube-like. In some cases the cavity of the tube is occupied by a dark looking substance formed of angular cells, and known as the _Medulla_. The walls of the tube, or the main substance of the hair, is made up of a pigmented, _h.o.r.n.y, fibrous material_. This fibrous structure is covered by a delicate layer of finely imbricated scales, and is termed the _Hair Cuticle_.

The root of the hair, that portion within the follicle, has exactly the same formation save at its extreme end. Here it becomes enlarged into a k.n.o.b-like formation composed of soft, growing cells, which k.n.o.b-like formation fits over a vascular papilla projecting up in the bottom of the follicle.

We have already stated that the hairs are down-growths of the epidermis.

It follows, therefore, that the hair follicles, really depressions or cul-de-sacs of the skin itself, are lined by epithelial cells and connective tissue. So closely does the epidermal portion of the follicle invest the hair root that it is often dragged out with it, and is known as the _Root Sheath_. This is made up of an outer layer of columnar cells (_the outer root sheath_) corresponding to the Malpighian layer of the epidermis, and of an inner h.o.r.n.y layer, next to the hair, corresponding to the more superficial layer of the epidermis, and known as the _inner root sheath_.

The hair grows from the bottom of the follicle by a multiplication of the cells covering the papilla upon which its root is moulded. When a hair is cast off a new one is produced from the cells covering the papilla, or, in case of the death or degeneration of the original papilla, the new hair is produced from a second papilla formed in place of the first at the bottom of the follicle.

[Ill.u.s.tration: FIG. 24.--SECTION OF SKIN WITH HAIR FOLLICLE AND HAIR. _a_, The hair follicle; _b_, the hair root; _c_, the medulla; _d_, the hair cuticle; _e_, the outer root sheath; _f_, the inner root sheath; _g_, the papilla from which the hair is growing; _h_, a sebaceous gland; _i_, a sudoriferous gland.]

THE SEBACEOUS GLANDS are small saccular glands with their ducts opening into the mouths of the hair follicles. They furnish a natural lubricant to the hairs and the skin.

THE SUDORIFEROUS OR SWEAT GLANDS are composed of coiled tubes which lie in the deeper portion of the skin, and send up a corkscrew-like duct to open on the surface of the epidermis. They are numerous over the whole of the body.

[Ill.u.s.tration: FIG. 25.--LONGITUDINAL SECTION THROUGH NAIL AND NAIL-BED OF A HUMAN FOETAL FINGER.[A] _a_, The nail; _b_, the rete mucosum; _c_, the longitudinal ridges of the corium.]

[Footnote A: Seeing that the section is a longitudinal one, it would appear from the way the ridges cut that they are running transversely beneath the nail. Their extreme delicacy, however, prevents a single one showing itself along the length of the section, and their constant accidental cutting makes them _appear_ to run transversely (H.C.R.).]

THE HUMAN NAILS are thickenings of the lowermost layer of the h.o.r.n.y portion of the epidermis, the stratum lucidum. They are developed over a modified portion of the corium known as the nail-bed. The h.o.r.n.y substance of the nail is composed of clear h.o.r.n.y cells, and rests immediately upon a Malpighian layer similar to that found in the epidermis generally. Instead of the papillae present elsewhere in the skin, the corium of the nail-bed is marked by longitudinal ridges, a similar, though less distinct, arrangement to that found in the laminae of the horse's foot.

Having thus paved the way, we are now in a better position to discuss our original question (Are the h.o.r.n.y laminae secreted by the sensitive?), and better able to appreciate the work that has been done towards the elucidation of the problem.

A most valuable contribution to this study is an article published in 1896 by Professor Mettam.[A] Here the question is dealt with in a manner that must effectually silence all other views save such as are based upon similar methods of investigation--namely, histological examination of sections of equine hoofs in various stages of foetal development.