268 DISEASES OV BONE. 



As a great rule, it is a malady o'f old, or at least, middle-age ; and, 

 wlieu it occurs, often follows an injury tO' the part, or chill. The 

 essential cause of the disease is indeed unknown. It appears to 

 have no connection with tubercle, gout, or true rheumatism except 

 in so far as these act as depressing agents. The bones that form 

 a joint, as explained on p. 256, do not come in contact during 

 health, but are kept apart by the articular cartilages which respec- 

 tively cover the ends of the bones, and which thus form two smooth 

 surfaces that being lubricated by synovia (joint oil) work on each 

 other without injury. The starting point of the disease appears in 

 most cases to be the articular cartilage. The first change observed 

 is a loss of polish and smoothness, gradually increasing till the 

 surface becomes velvety in appearance. Microscopic examination 

 shows that this change is due to the gradual conversion of the 

 matrix into fibrous tissue, the fibrillse of which are arranged at right 

 angles to the surface. 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 impunity, the 

 surfaces which, during movement of the joint, now rub against 

 each other, become inflajned, and a deposit of very compact bone 

 is formed in their substance. When the opposing surfaces of bone 

 have assumed this ivory-like or porcellaneous structure, 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 cause a deposit of bony material about the diseased 

 part. From the foregoing remarks, it may be assumed, 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, two, if not three, forms of joint inflammation can 

 be recognised, closely resembling rheumatoid arthritis of man. 

 They may be described as follows : — 



(1.) The not very uncommon chronic inflajnmation of tlie 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- 

 veloped 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 perfectly 



