DISEASES OF THE JOINTS 435 
Even with the limb at rest the pressure on both sides of 
the navicular bone is still constant. The only circum- 
stances under which we can conceive of it being entirely 
absent, in fact, are when the tension on the tendon is 
relaxed, and the body-weight altogether removed by the 
animal adopting the recumbent position. 
The compression theory as to the causation of navicular 
disease was, we believe, first originated by Colonel Smith. 
He, at any rate, has laid much stress on it in hig writings. 
If we accept it, and we see every reason that we should, 
then we must, with the author, admit the possibility of 
navicular disease arising from long standing in one position. 
3. Concussion.—This we are bound to admit as a cause, 
and in so doing partly explain the comparative, almost total, 
immunity of the hind-feet from the disease. The fore- 
limbs, as we have already pointed out, are little more than 
props of support, and the force of the propelled body-weight 
is transmitted largely down their almost vertical lines, to 
end largely in concussion in the foot. With the hind-limbs 
matters are different. ‘These,’ as Percival explains it, 
‘have their bones obliquely placed, so as to constitute, one 
with the other, so many obtuse angles, to the end, that by 
forming powerful levers, and affording every advantage for 
action to the muscles attached to them, they may be fitted 
for the purpose of propulsion of the body onward.’ 
The effect of these several obtuse-angled joints in the 
limb is to absorb the greater part of the force exerted by 
the body-weight before it reaches the foot. When with this 
we take the facts that the fore-limbs have to carry the head 
and neck, and that they have to bear this added weight, 
plus a propelling force from behind, we see why it is that 
they should be so subject to the disease, and the hind-limbs 
so exempt. 
As pointing out the part that concussion plays in its 
causation, we may mention that navicular disease is a disease 
of the middle-aged and the worked animal. Itis interesting 
to note, too, that it occurs in animals with well developed 
frogs—in feet in which frog-pressure with the ground is 
28—2 
