mechanical interference may occur to the navicular circulation.  He further points out a fact that is, of course, well known to every veterinarian, that in periods of work the circulation of the foot is hurried, and that in rest there is always a tendency to congestion; and he says in conclusion:  ’I cannot help thinking that irregularities in the blood-supply in a naturally weak bone must be a factor of some importance, especially when the kind of work the horse is performing is a series of vigorous efforts followed by rest.’

6. Senile Decay.—­With approaching age the various tissues lose their vigour, and are prone to disease.  The navicular bone and surrounding structures are not exempt.  With the other and more active causes we have described acting at the same time it is not surprising that navicular disease is seen as a result.

In conclusion, it is well, perhaps, to say that, no matter to which particular theory of causation we may lean, we should make up our minds to consider them as a whole.  While one cause may be exciting, the other may be predisposing, and the two must act together before evil results are noticed.  It may be that even more than two are concerned in bringing on the disease, and to each the careful veterinarian will give due consideration.

Symptoms and Diagnosis.—­In the early stages of navicular disease the symptoms are obscure.  Pointing of the affected limb is the first evidence the animal gives.  This, however, more often than not, goes unnoticed, and the first symptom usually observed by the owner or attendant is the lameness.  Even this is such as to at first occasion no alarm, being intermittent and slight, and only very gradually becoming marked.  In a few cases, however, lameness will come on suddenly, and is excessive from the commencement.  It is the lameness, slow in its onset, intermittent in its character, and gradual in its progress, however, that is ordinarily characteristic of navicular disease.

The animal is taken out from the stable sound, with just a vague suspicion, perhaps, that he moved a bit stiffly.  While out he is thought by his driver or rider to be going feelingly with one foot or with both.  Even this is not marked, and the driver has some difficulty in assuring himself whether or no he really observed it, or whether it was but imagination.

On the return home the limb is examined, and nothing abnormal is to be found.  The leg is of its normal appearance, and neither heat nor tenderness is to be observed in it or in the foot.  On the following day the animal again is sound, and the lameness of the previous day is put down to a slight strain or something equally simple.  The patient is then, perhaps, rested for a day or two.  When next he is worked he again moves out from the stable sound, but again during the going gives the driver the unpleasant impression that something is amiss; and so the case goes on.  One day the owner fears the animal is becoming seriously enough affected to warrant him in calling in his veterinary surgeon; the next he is confidently assuring himself that nothing is wrong.