270 DISEASES OF BONE. 



to the surface " {Erichse^i). Owing to loss of smoothness, friction 

 is set up, with the result that the opposing and altered cartilages 

 become worn away, and the ends of the bones are brought into 

 contact. As exposed bone is unable to bear friction with im- 

 punity; the surfaces which, during movement of the joint, now 

 rub against each other, become inflamed, and a deposit of very 

 compact bone is formed in their substance. When the opposing 

 surfaces of bone have assumed this ivory-like or porcelaneous struc- 

 ture, the joint, when moved, will naturally be liable to emit a 

 creaking sound, which is a frequent symptom of the disease in an 

 advanced stage. The effect of the inflammation, if long continued, 

 is to more or less enlarge and deform the ends of the bones 

 which form the joint, and to cause a deposit of bony material 

 about the diseased part. From the foregoing remarks I think 

 we may assume, at least for convenience sake, that rheumatoid 

 arthritis is an inflammation of articular cartilage, followed by 

 destruction of the opposing portions of cartilage, and by the deposit 

 of peculiarly dense bone on the exposed bony surfaces. 



In the horse, we can recognise two, if not three, fomis of joint 

 inflammation closely resembling rheumatoid arthritis of man. 

 They may be described as follows : — 



(1.) The not very uncommon chronic inflammation of the true 

 hock joint which is characterised by pain ; lameness ; the emission 

 of a creaking sound, especially when the animal begins to move 

 after rest; often, if not always, by an increased amount of fluid 

 in the joint; and by bony deposit about the part. The course of 

 this disease in the horse appears to be identical with that of 

 rheumatoid arthritis of man. As a great rule, it becomes de- 

 veloi>ed only in old or at least adult horses which have done a 

 good deal of hard work. It is only fair to say that some hocks which 

 are perfectly healthy emit at times a similar creaking sound, which, 

 therefore, cannot be regarded as peculiar to this disease ; although, 

 possibly, it may be a constant symptom. There is always a certain 

 amount of stiffness in the joint, with, at times, marked lameness. 

 In some cases, I have seen, while the horse was at work, sudden 

 and most painful lameness occur, even to the extent of the animal 

 holding up the affected limb off the ground, for a short time. 

 This seizure would pass off after a few minutes' rest. The cause 

 of it, I presume, was the detachment, from the diseased cartilage, 

 or from the bony deposit, of a particle which got between the 

 articulating surfaces. 



A horse with this disease in the hock may continue capable of 

 a fair amount of work for years. 



(2.) A similar though more acute form of inflammation of the 

 stifle joint. I have met with one case of this in a van (light cart) 



