G. THE COMPLEMENTARY APPARATUS OF THE OS PEDIS.

This consists of two lateral pieces, the LATERAL CARTILAGES or Fibro-cartilages of the pedal bone, united behind and below by the Plantar Cushion.

1.  THE LATERAL CARTILAGES.—­Each is a flattened plate of cartilage, possessing two faces and four borders separated by four angles.

The external face is convex, covered by a plexus of veins, and slightly overhangs the pedal bone.  The internal face is concave, and covers in front the pedal articulation and the synovial sac, already mentioned as protruding between the antero- and postero-lateral ligaments of that joint.  We have already remarked that this is a point of interest to be remembered in connection with the operation for quittor.  Below and behind, the internal face of the cartilage is united to the plantar cushion.

[Illustration:  FIG. 16.—­EXTERNAL FACE OF THE OUTER LATERAL CARTILAGE. 1, External face of cartilage—­(a) its upper border, (b) its posterior border, (c) its anterior border, (d) its inferior border; 2, the os pedis; 3, wing of os pedis.]

The upper border, sometimes convex, sometimes straight, is thin and bevelled, and may easily be felt in the living animal.  It is this border that the digital vessels cross to gain the foot, and the border is often broken by a deep notch to accommodate them.  The inferior border is attached in front to the basilar and retrossal processes, behind which it blends with the plantar cushion.  The posterior border is oblique from before to behind, and above to below, and joins the preceding two.  The anterior border is oblique in the same direction, and is intimately attached to the antero-lateral ligament of the pedal articulation.  The cartilages of the fore-feet are thicker and more extensive than those of the hind.

2.  THE PLANTAR CUSHION on FIBRO-FATTY FROG.—­Composed of a fibrous meshwork, in the interstices of which are lodged fine elastic and connective fibres and fat cells, this wedge-shaped body occupies the space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog.  It offers for consideration an antero-superior and an infero-posterior face, a base, an apex, and two borders.

The antero-superior face is in contact with the terminal expansion of the perforans tendon.  The infero-posterior face is covered by the keratogenous membrane, and follows closely the shape of the horny frog, on whose inner surface it is moulded.  It presents, therefore, at its centre a single conical prolongation, the Pyramidal Body, which is continued behind, as is the horny frog, in the shape of two lateral ridges divided by a median cleft.  The base of the cushion lies behind, and consists of two lateral masses, the Bulbs of the Plantar Cushion.  In front these are continuous with the ridges of the pyramidal body, while behind they become confounded with the lateral cartilages and the coronary cushion.  The apex is fixed into the plantar surface of the os pedis, in front of its semilunar ridge.  The borders, right and left, are wider behind than before, and are in relation with the inner faces of the lateral cartilages.