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 circumference.  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 circumstances, its deepest part being at the point of the frog.  Sloping from this point to its circumference, it becomes suddenly flat just before joining the wall.  Its horn in appearance is flaky.

[Illustration:  FIG. 21.—­INFERIOR ASPECT OF HOOF. a The inferior face of horny 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 Circumference is intimately dovetailed with the horny laminae of the wall.  At its circumference 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 horny 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.