DISEASES OF THE JOINTS 279 



The joints below the hock are the same as those below the knee, 

 which have already been described in considering the articulations of the 

 fore-limb. 



DISEASES OF THE JOINTS 



As we have elsewhere pointed out, joints vary very considerably, not 

 only in their structure, but also in the purpose they serve. Some, as those 

 by which the bones of the head are united, are fixed and immovable, while 

 those of the extremities enjoy a considerable range of action, and upon their 

 liberty or freedom of movement depends the power of locomotion. The 

 former are formed solely by two bones held together by the interlocking of 

 their serrated edges, while the latter comprise not only two or more bones 

 whose articular surfaces are encrusted with cartilage, but these are enclosed 

 in a synovial membrane spread over the internal surface of a sac termed 

 the capsular ligament. In addition, strong bands of fibrous tissue, in the 

 form of "connecting ligaments", unite the bones together. This form of 

 joint is known as the diarthrotic, and is the most free-moving of the several 

 varieties. 



A third variety of joint is found uniting the bodies of the vertebrae. 

 Here, between the bones, a dense cushion of fibrous tissue is interposed. 

 It has no synovial or capsular membrane, as in the case of the diarthrotic 

 variety of joint, and its range of movement is very restricted, being 

 only so much as to impart to the spine a limited degree of flexibility and 

 strength. This is known as an amphiarthrotic joint. 



Of the three varieties of joints above referred to, it is very seldom that 

 disease is found to exist in any but those of the diarthrotic class, and in 

 this section the joints of the extremities afford by far the greater number 

 of examples. Their larger size, more exposed position, great range of action, 

 and liability to jar, concussion, and sprain, are no doubt the factors which 

 tell in favour of disease. 



Joint diseases are brought into existence either by local or constitutional 

 conditions, or both, as when the latter predispose to the influence of the 

 former. 



If there is one period of life when horses suffer more from joint disease 

 than another, it is from birth up to six years old, when the burdens they 

 have to bear are badly apportioned to their condition and strength, and 

 when certain constitutional ailments which determine these affections are 

 most prevalent. 



External violence is perhaps the most common exciting cause of joint 

 disease. Kicks from other horses, slips, falls, blows, sprains, and penetrating 

 wounds by which inflammation more or less severe is excited in one or 



