THE SKELETAL SYSTEM 



51 



elbow; and ball-and-socket joint, as that between the pelvis 

 and the femur at the hip. 



The freely movable joints are the only true joints (Fig. 9). 

 They are composed of the following structures : 



1. Two or more bones with joint surfaces that are usually 

 somewhat expanded. 



2. Articular cartilages, which 

 cover the articular surfaces of the 

 bones. These are exceedingly 

 smooth and are thickest on the 

 central part of a convex surface, 

 while on a concave surface the 

 marginal cartilage is thickest. 

 They diminish the effects of con- 

 cussion and greatly reduce friction. 



3. A joint capsule, which re- 

 sembles a sac open at both ends, 

 is attached around the articulating 

 surfaces. It is composed of two 

 layers — an external, strong fibrous 

 layer, and an internal, velvety 

 synovial layer. 



4. Ligaments, strong fibrous 

 bands which bind the bones firmly 

 together. They are pliable, but 

 inelastic, and maintain the articular surfaces in more or less 

 close contact and thus greatly strengthen the joint. In most 

 joints the ligaments lie external to the joint capsule. In others, 

 such as the hip and stifle, they lie within the joint-capsule. 



Synovia is a viscid, straw-colored fluid which gives an alkaline 

 reaction. It is not secreted by glands but is a tissue fluid that 

 exudes into the joint. It contains proteins, salts, and mucin. 

 Synovia reduces friction by lubricating the joints. The amount 

 of synovia increases and diminishes to a certain extent under 

 normal conditions. When an excessive quantity is present a 

 puffy swelling occurs in the region of the joint. Bog spavin is a 

 good example of an excess in the hock-joint. 



All joint movements are due to muscle contraction and relaxa- 

 tion. True joints are found to vary greatly in their possibilities 

 for movement depending upon the location and extent of their 

 ligaments. The se^nYjgg^^jp^^^g^ joint movements are: 



Fig. 9. — Diagram of section 

 of true joint: f.l., Fibrous layer; 

 s.L, synovial layer of joint cap- 

 sule. The articular cartilages 

 are white, bones dotted, and 

 joint cavity black in the figure. 

 (Sisson, Anatomy of Domestic 

 Animals.) 



