58 SPRAINS. 



Sprain of the Shoulder. 



ANATOMY.— The shoulder joint (Fig. 31) is formed by the shoulder- 

 blade (Fig. 115) and humerus. This ball-and-socket joint being required to 

 possess great power of extension and flexion, as well as some side play, is 

 bound down by only one ligament (the capsular) ; the office of other liga- 

 ments being taken by the tendons of certain muscles which pass over it, and 

 thus help to keep it in place. A large muscle (the Aexor brachii) takes 

 its origin at the head of the shoulder-blade, passes over the shoulder joint, 

 and is attached to the head of the radius. Its action is to straighten the 

 shoulder joint and raise the knee. The shoulder joint is kept close to the 

 side by the pectoral muscles, which connect it, above and below, to the 

 body, and thus prevent it from bulging outwards. Two of these muscles 

 (the deep pectorals) act in drawing the point of the shoulder backwards and 

 downwards. 



NATURE. — The shoulder is so large, and its component parts 

 so concealed from sight and touch, that, in the case of shoulder 

 lameness, it is often very difficult to locate the seat of injury. 

 Formerly, sprain of the pectoral muscles was regarded as the most 

 frequent cause of this lameness, owing to the fact that in such 

 cases these muscles were unable to perform their usual function of 

 keeping the shoulder joint close to the body. This muscular in- 

 ability is also seen in fracture of the first rib (p. 308), which is a 

 frequent cause of shoulder lameness. There may be also sprain of 

 the flexor brachii, of the capsular ligament, and injury to the 

 nerves of the part, or to the joint itself. 



CAUSES. — The usual causes are : (1) over-exertion of the pec- 

 toral muscles ; this is s:pecially liable to occur with young horses 

 when ploughing, to which they are put, as a rule, at an earlier age 

 than to any other kind of labour ; (2) wrenches when turning, and 

 slips, by which the shoulder is pulled forcibly outwards and some- 

 times forwards, in direct antagonism to the action of the pectoral 

 muscles ; and (3) falls or blows, by which the joint and its neigh- 

 bouring parts may be injured. 



SYMPTOMS. — When the pectoral muscles are affected, the point 

 of the shoulder is seen to bulge out at each step, and the foot is 

 swung outwards when brought to the front, as in fracture of the 

 first rib (p. 308). When the flexor brachii is hurt, the animal will 

 naturally " drag " the leg from inability to raise it. The flexor 

 brachii extends the shoulder joint, when raising the leg ; unwilling- 

 ness, to lift the leg up will therefore be shown, when movement of 

 the shoulder joint is accompanied with pain, as in injury to the 

 capsular ligament, or to the joint. In neither sprain of the pec- 



