50 THE HORSE IN HEALTH AND DISEASE 



2. The articular cartilages which cover the articular surfaces 

 of the bones. They are exceedingly smooth and are thickest on the 

 central part of a convex surface, while on a concave surface the 

 marginal cartilage is thickest. These cartilages diminish the effects 

 of concussion and greatly reduce friction. 



3. The joint capsule is a sort of sac, the margins of which are 

 attached around the articulating surfaces. It is composed of two 

 layers — an external, strong fibrous layer, and an internal synovial 

 layer, which secretes the synovia or "joint-oil" to lubricate the 

 joint. 



4. The ligaments are strong fibrous bands which come down on 

 each side outside the joint capsule and bind the bones firmly to- 

 gether. They are pliable, but inelastic, and maintain the articular 

 surfaces in more or less close contact and thus greatly strengthen 

 the joint. 



The joint cavity is a potential cavity and does not actually 

 exist in the sense of a space occupied by air. When the bones of 

 the articulation are forcibly pulled apart there may be a small 

 space, but it is largely filled with the synovia. 



Movements of True Joints. — As a class the freely movable 

 joints have a variety of movements. When considered separately, 

 joints of this type are found to vary greatly in their possibilities 

 for movement. There are seven principal kinds of joint move- 

 ments : 



1. Gliding, the simplest and only movement possible between 

 two plane or undulating facets. 



2. Flexion, which brings two bony pieces nearer each other by 

 closing the angle more or less. 



3. Extension, the inverse movement by which the bones are 

 straightened on each other. 



4. Adduction, which brings the inferior end of the movable bone 

 toward the median plane of the body. 



5. Abduction, where it has the opposite effect. 



6. Circumduction, in which the shaft of the distal bone of the 

 joint describes the surface of a cone. 



7. Rotation, in which one bone pivots or rotates on the other. 

 From a practical standpoint, the shoulder, fetlock, pastern, and 



coffin joints of the fore leg, and the hip, stifle, and hock joints of the 

 hind leg are the most important. We have already learned about 

 the shape of the articular surfaces of the bones which enter into 



