DISEASES OF THE JOINTS 435 



Even with the limb at rest the pressure on both sides of 

 the navicuh^r 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 his 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. Coiicussio)i. — 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. It is 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 



