When an enlargement appears on the point of the hock, the part is said to be "capped" (fig. 386). There are two conditions to which this term is applied - one involving the skin and tissue beneath it, and the other the synovial membrane interposed between the bone (calcis) and the tendon passing over it.
A short description of the anatomy of this part will render this difference intelligible. The point of the hock comprises a bony prominence, over which is spread an expanded portion of the tendon of the gastrocnemius in-ternus muscle, and in order that the latter may more freely move over the former, a synovial sac is interposed between the two and supplies the necessary fluid to lubricate the apposed surfaces. On the posterior aspect of the tendon there is a considerable quantity of cellular tissue, which by its looseness enables the skin to move freely over the point of the hock, and thus to accommodate itself to the extremes of flexion and extension during progression.
Injury to this cellular structure results in inflammation (cellulitis) and swelling, which, when attended with an accumulation of fluid in the part, constitutes a serous abscess.
The other form of the disease is always one of distension of the synovial sac with synovial secretion; but inasmuch as it does not concern the summit of the hock, but only appears at the sides, the term "capped hock" ought never to have been applied to it.
Whether the enlargement be of one description or the other, it is invariably the result of external violence. The most common cause is injury done to the point of the hock in lying down on a badly littered floor, or by slipping and striking the part in the act of rising. Pitching on the hocks when falling backwards in rearing is sometimes the cause, and in many cases it is referable to kicking. The slighter cases of this disease result from rubbing the points of the hocks repeatedly against the wall or stall-posts while rubbing the tail.
Fig. 386. - Capped Hock.
Of the two forms of the malady, that arising out of injury to. the subcutaneous cellular tissue is by far the more common, and, it may also be said, the less important.
Capped hocks usually appear suddenly. The size and nature of the swelling will vary with the manner of its production. Where it is excited by repeated rubbing the growth is small at first, and gradually increases in size so long as the cause continues in operation. Blows inflicted in any of the several ways indicated above are followed by sudden swelling, which will be proportionate to the injury. Sometimes it is very considerable, hot and painful to the touch. In consistence it may be firm or tense and fluctuating like an inflated bladder. The latter conditions indicate the presence of fluid beneath the skin, which mostly consists of blood-stained serum (serous abscess).
It is very seldom that lameness results from this form of the disease, although it sometimes occasions slight stiffness for a few strides when recently produced.
The second and exceptional form of the disease is readily distinguished from the first by the fact of the enlargement appearing at the sides of the point of the hock and not on the point itself. It is, moreover, always in the nature of a fluctuating swelling, and, as a rule, inconsiderable in size. It not infrequently gives rise to lameness, and when complicated with disease of the bone is troublesome and serious.
Slight cases of true capped hock do not require much treatment. Cold-water irrigation and cold sponging for a few days, coupled with removal of the cause, and, later, a little hand rubbing daily with gentle pressure, will suffice. Where, however, the swelling is considerable and the hock much inflamed, a dose of physic should be administered at once, and hot fomentations applied freely to the injured part; with the decline of the inflammation, cold-water irrigation may be substituted for hot fomentation. Daily exercise or light work will be beneficial rather than otherwise when the tenderness has disappeared, and a little gentle hand rubbing with pressure may aid in removing the swelling. Where the enlargement proves obstinate a mild blister may be applied and repeated if necessary. It frequently happens in the more severe cases that a permanent and unsightly enlargement remains after all has been done; but there is a tendency to diminish in size as time goes on, so long as no further injury is permitted to be inflicted. Where the enlargement assumes the form of a serous abscess of considerable size, the fluid may be removed by means of an aspirator or by subcutaneous puncture.