DISEASES OF THE JOINTS 437 
term a weak navicular bone. In this connection Colonel 
Smith says: ‘Though it is far from present in every case 
of the disease, still I consider it a factor of great importance.’ 
5. A Defective or Irregular Blood-supply to the Bone— 
- This, Colonel Smith considers, is brought about by excessive 
-and irregular work, and by the opposite condition—rest. 
The author points out that the bloodvessels passing to and 
from the navicular bone run in the substance of the inter- 
osseous ligaments, or in such proximity to them that it is 
conceivable that under certain circumstances 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‘ series of vigorous efforts followed by rest.’ 
6. Senile Decuy.— 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 pre- 
disposing, and the two must act together before evil results 
are noticed. It may be that even’ more than two are con- 
cerned 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. 
