The horn of the wall, viewed immediately from the front, is known as the Toe, which again is distinguished as Outside Toe or Inside Toe, according as the horn to its inner or outer aspect is indicated. The remainder of the external face of the wall, that running back to the heels, is designated the Quarters.
In the middle region of the toe, the wall following the angle of the bones is greatly oblique. This obliquity decreases as the quarters are reached, until on reaching the heels the wall is nearly upright.
FIG. 19.—THE WALL OF THE HOOF. 1, The toe; 2, inner toe; 3, outside toe; 4, the quarter; 5, entigeral groove; 6, horny laminæ.
For observation the wall offers two faces, two borders, and two extremities.
The External Face is convex from side to side, but straight from the upper to the lower border. Examined closely, it is seen to be made up of closely-arranged parallel fibres running in a straight line from the upper to the lower border, and giving the surface of the foot a finely striated appearance. In addition to these lines, which are really the horn tubules, the external face is marked by a series of rings which run horizontally from heel to heel. These are due to varying influences of food, climate, and slight or severe disease. This will be noted again in a later page. In a young and healthy horse the whole of the external face of the wall is smooth and shining. This appearance is due to a thin layer of horn, secreted independently of the wall proper, termed the periople.
FIG. 20.—INTERNAL FEATURES OF THE WALL, FROG, AND SOLE (MESIAN SECTION OF HOOF). 1, Horny laminæ covering internal face of wall; 2, superior border of wall; 3, junction of wall with horny sole; 4, the cutigeral groove; 5, the horny sole; 6, the horny frog (that portion of it known as the 'frog-stay'); 7, inverted V-shaped ridge on wall and sole (known as the 'toe-stay'); 8, anterior face of wall; 9, inferior border of wall.
The Internal Face of the wall, that adapted to the sensitive laminæ, is closely covered over its entire surface with white parallel leaves (Keraphyllæ, or horn leaves, to distinguish them from the Podophyllæ, or sensitive leaves). These keraphyllæ dovetail intimately with the sensitive laminæ, covering the os pedis. Running along the superior portion of the inner face is the Cutigeral Groove. This cavity has been mentioned before as accommodating the coronary cushion, whose shape and general contour it closely follows, being widest and deepest in front, and gradually decreasing as it proceeds backwards. It is hollowed out at the expense of the wall, and shows on its surface numberless minute openings which receive the papillæ of the coronary cushion.
At the bottom of the internal face, at the point where the toe joins the sole, will be noted the before-mentioned inverted V-shaped prominence. Its position will be clearly understood when we say that it gives the appearance of having been forced there by the pressure of the toe-clip of the shoe. This will be noted again when dealing with the sole.