The ‘feeling’ manner of the gait before alluded to, together with the disinclination to put the foot firmly and squarely forward, will sometimes lead the examiner to over-look the contraction, and diagnose his case as one of shoulder lameness.  In many cases, too, such consequent conditions as ‘thrushy frogs’ and ‘suppurating corns’ are often treated with utter disregard of the contraction that has really brought them about.  But above all, the disease most likely to be confounded with simple contraction is navicular disease.  More than probable it is that many cases of so-called ‘navicular’ have in reality been nothing more than contraction brought about by one or other of the causes we shall afterwards enumerate—­cases where a due attention to the prime cause of the mischief would, in all likelihood, have remedied the lameness.

Changes in the Internal Structures.—­It follows as a matter of course that the changes we have described in the form of the hoof itself carry with them alterations in the bones and sensitive structures beneath it.  The tissues, as a whole, become atrophied.  The os pedis becomes deformed, loses its circular shape, and gradually becomes more or less oval in contour.  At the same time, its structure becomes more compact, the cribriform appearance of its anterior and lateral faces more or less destroyed, and the few remaining openings apparently increased in size.  This atrophy of the os pedis is best noted at the wings.

In the plantar cushion the effects of the atrophy are noted in the smallness of the organ, in its becoming whiter in colour than normal, and more resistant to pressure.

The coronary cushion is also affected in the same way, where the changes are noted most in its posterior portions.

A further effect of the narrowing of the heels, and their consequent tendency to drop downwards, is the exertion of a continual pressure on the sensitive sole.  In course of time, and especially in flat feet, this leads to the appearance of corns.

The navicular bone and bursa and the tendon of the perforans also suffer from the effects of compression.  The movement of the tendon is restricted, and arterial supply to the adjacent structures rendered deficient.  The tissues of the bone and bursa are insufficiently nourished, and the secretion of synovia lessened.  In this way it is conceivable that navicular disease may follow the condition of simple contracted heels.

In common with the other structures, the lateral cartilages also suffer from the continual pressure.  Their blood-supply is lessened, their functions interfered with, and side-bones result.

Causes.—­Upon the causation of contraction a very great deal has been written, both by early veterinarians and by those of the present day.  Many and widely differing opinions have been advanced, but a careful resume of only a few will lead one to certain fixed conclusions.