HdRSE's. 1 35 



tally across the foot, excepting that it is in the form of a crescent, 

 exactly resembling a very small shuttle. It is called the shuttle- 

 bone, or from its resemblance to the shape of a boat, the navicular- 

 bone. The outer side of the crescent lies backwards and down-- 

 wards, and has a ridge in the centre of its surface. It is articulat- 

 ed both with coronet-bone and with the coffin-bone. Its especial 

 office is to form an additional pulley for the back-sinew ; which 

 passed over its convex side, and is kept in its proper place upon it, 

 by a groove corresponding to the ridge upon the bone. A bursa 

 mucosa, or sac, containing and secreting the sinovia or joint oil, is 

 interposed between the back-sinew and the bone ; no way differ- 

 ing. I take it, in office or liability to disease from the bursas mu- 

 cosae in other parts of the horse's frame. All the weight which 

 the horse throws upon the leg is of course received by the coronet- 

 bone, which, being articulated with the coffin-bone, which when 

 the hoof is placed upon the ground, can have no motion but that 

 allowed by the cartilages and the laminas at its sides, sinks back- 

 wards and downwards, from its joint with the pasten-bone, directly 

 upon the navicular bone. The navicular-bone being articulat«a 

 with the coffin-bone, which is aearly motionless, receives what 

 weight is not thrown upon the coffin-bone, rolls backwards and 

 downwards also upon the back-sinew. The back-sinew being fast 

 immediately almost below, and being perfectly inelastic, if the na- 

 vicular-bone cannot roll upon it, the joint, if it may be called so, is 

 destroyed. The weight is now transferred to the back-sinew, and 

 is partly received by the muscles into which it is inserted above, 

 and they share it with the elastic internal frog below the back- 

 sinew, and the back part of the foot generally ; all of which, if the 

 horse has never been shod, is highly elastic. As this joint is the 

 most perpendicularly opposed to the resistance of the earth of any 

 joint in the horse's frame, and as all the elasticity of the back part 

 of the foot, which Nature calculates upon, is neutralized by the 

 iroii, and in fact its area much diminished in general, disease of it 

 would really seem to be very often expected in a shod horse, ex- 

 posed to the concussion of fast work upon a hard road. That it 

 seldom or never occurs in the hind feet is natural, when we con- 

 sider that the weight thrown upon them is comparatively trifling," 

 and that the resistance of the earth is not opposed to it perpendicu- 

 larly, but in a very oblique direction. That foot-lameness general- 

 ly arises immediately from concussion, is almost proved from its 

 never occurring in the hind feet, which are also exposed to strains : 

 and that it generally arises from disease of this joint is supported 

 by the feet of this joint's being most exposed of any in the foot to 

 concussion in the fore (eet. 



This joint, therefore, appears to be very much exposed to in- 

 flammation from direct injury, or if predisposed to it, it may ap- 

 propriate to itself any general inflammation of the foot, from what- 

 ever cause. The first step of the disease is in the bursa mucosa^ 

 or sac, between the bone and the back-sinew ; the second is a de- 

 struction of the smooth surface, and a caries of the bone ; a conse- 

 quent impediment to the roll of the bone upon t!m bi^ck-slnew : 



