262 HEALTH AND DISEASE 



Kicks, blows, and violent struggling when cast are the most common 

 causes. In foals the point of the hock is sometimes torn off when rearing 

 and walking backwards on the hind-legs. 



The symptoms resulting from this mishap are very striking and 

 characteristic. 



There is a complete inability to support weight on the broken limb. 

 When an attempt is made to do so the hock joint sinks towards the 

 ground, and the point of the hock is drawn upwards by the pull of the 

 tendo achillis, the foot is advanced, and the quarter on the injured side 

 inclined downward. In progression the limb as a whole is raised as far 

 as possible, and then trailed forwards with the advancing body. 



Treatment in these cases offers but little prospect of success. The 

 tendo achillis, acting upon the broken fragment, displaces it upward beyond 

 readj ustment, and where it is practicable to bring the broken parts together 

 it is usually found impossible to maintain them in their natural position. 



Fractures below the hock may be such as have been described as taking 

 place below the knee, and will require to be healed on the same lines. 



16. ARTICULATIONS OR JOINTS 



The bones of the skeleton are joined together in various ways to form 

 joints. The manner in which they are united will depend upon the 

 purpose they are intended to perform, hence joints are divided into three 

 classes according to their respective range of movement. 1. Diarthrodial 

 joints, which enjoy the greatest freedom of action. 2. Amphiarthrodial 

 joints, whose powers of movement are much more restricted. 3. Syn- 

 arthrodial, or those which are fixed and immovable. 



Diarthrodial or Free-moving Joints are composed of two or more 



bones whose articular surfaces are covered with a thin layer of cartilage 

 or gristle, and so formed as to permit one to play freely upon the other. 

 They are all enclosed in a sac lined by a delicate membrane for the 

 secretion of synovia or joint-oil, and for the most part the bones are 

 held together by connecting ligaments. 



Ball-and-Socket Joints. Some diarthrodial joints are formed by 

 the rounded head of one bone fitting into a cup-like cavity or socket con- 

 tained in another bone. This is the case in the hip-joint (fig. 358), which 

 allows the leg to be moved in all directions inward, outward, forward, 

 backward and also to be rotated and circumducted or moved in a circle. 



Hinge Joints. Others assume the form of hinge joints, in which 



