In other cases horn in this position is altogether wanting, and in its place is a well-defined cavity, into which the blade of a knife can be readily passed.  This cavity is bounded in front by the original wall of the hoof, and is here lined by a degenerated and hypertrophied growth of the horny laminae.  Posteriorly the cavity is bounded by the front of the os pedis, and is lined by a thin growth of horn secreted by the keratogenous membrane covering the bone.  Superiorly the cavity is quite narrow, and extends to near the lower surface of the coronary cushion, while inferiorly, at its open portion, it is often 1/2 inch to 1 inch wide.  Laterally it extends on each side of the toe to the commencement of the quarters.

[Illustration:  FIG. 124.—­LONGITUDINAL SECTION OF A FOOT WITH LAMINITIS OF THREE WEEKS’ STANDING.  On the anterior face of the cavity, in front of the os pedis, are thickened horny laminae.  Due to the sinking of the bony column, the os pedis has perforated the horny sole.]

Exploration with a director, or with the blade of a scalpel, removes from the opening a dry detritus.  This is composed of the solid constituents of the escaped blood, the dried remains of the inflammatory exudate, and broken-down fragments of cheesy-looking horn.  The size to which the cavity may sometimes extend is illustrated in Fig. 124.  The thickened horny laminae forming the anterior boundary of the cavity are here depicted, together with commencing perforation of the horny sole by the os pedis.  It is this cavity which, when opened at the bottom and discharging its mealy-looking contents, is known as seedy-toe, for a further description of which see p. 293.

The lameness occurring with chronic laminitis does not always persist.  As time goes on the sensitive structures accommodate themselves to the altered form and conditions of the horny box.  In certain situations—­namely, where pressure is greatest—­the softer structures become atrophied, and sometimes even wholly destroyed; while in other positions the changes in form of the hoof tend to increase in size of its interior, with a consequent diminution of pressure upon, and increased growth of the structures within it.

Pathological Anatomy.—­In detailing the changes to be observed in chronic laminitis, we take up the description where we left it when dealing with the pathological anatomy of the acute form.  The alterations to be met with are best observed by taking a foot so diseased and making of it two sections—­one longitudinal, from before backwards; the other horizontal, and in such a position as to cut the os pedis through at its centre.