504 
LUXATION OF THE SHOULDER-JOINT. 
and only allow slight abduction, adduction, and rotation, though considerable 
flexion and extension. Flexion of the shoulder-joint is limited by the biceps, 
extension by the caput muscles, abduction by the subscapularis, adduction 
by the antea and postea spinati, so that the shoulder-joint in domesticated 
animals possesses far less mobility than in man. The oval flat glenoid cavity 
is much smaller than the head of the humerus. In swine and carnivora, the 
size of the former is increased by a cartilaginous disc (Labrum glenoideum 
scapulae). In these animals the shoulder-joint is also connected with the 
bursa intertubercularis. 
(a) Luxation of the Shoulder-Joint.—Incomplete luxation of the 
shoulder is very rare in animals, there being nothing to keep the head 
of the joint in position on the sharp edge of the glenoid cavity. Com¬ 
plete displacement is also much rarer than in man, though it occurs 
both in horses, ruminants, and dogs. In swine and carnivora, the 
position of the caput humeri is ensured by the labrum glenoideum; in 
the horse, by the tendinous biceps muscle. 
Luxation can generally be referred to excessive flexion of the joint, and 
the humerus is, therefore, always thrust forwards and upwards, the head 
of the bone being discovered in front of and above the glenoid cavity of 
the scapula. As the biceps brachii muscle antagonises this movement, * 
displacement can only occur when that muscle is partially relaxed, as it 
would be, for instance, during simultaneous flexion of the elbow-joint. 
For this reason luxation of the shoulder-joint results most frequently 
from falls. A sudden check to the movement of the lower portions of the 
leg may also bring it about. Hertwig saw several such displacements, on 
account of the feet striking against some obstruction in leaping. Excessive 
movements of abduction may also result in the head of the humerus 
being displaced inwardly beyond the glenoid cavity of the scapula. 
Symptoms. The accident is distinguished by excessive lameness, 
which appears suddenly, and is most marked when the leg is being 
moved. Passive movement of the joint is also interfered with. Whilst 
there is much difficulty in flexing, and still more in extending, the joint, 
abduction and adduction are abnormally easy, and the leg appears 
shortened. Provided swelling be not excessive, dislocation of the head of 
the bone can be discovered by palpation. Pressure on the parts produces 
pain. Complications, like fracture of the scapula or humerus, often exist. 
Prognosis and course. The results of many observations seem to 
indicate that even complete luxation, if early reduced, can be cured in 
two to three .weeks. Smith, after reducing a supposed dislocation under 
chloroform, caused the horse to be moved, and it then showed no distinct 
lameness ; eight days later it was nearly sound. The diagnosis, how¬ 
ever, in this case is not beyond question. In other cases chronic lame¬ 
ness remains. In working-horses, it is best to wait for eight to fourteen 
days after reduction, and if in this time no improvement is manifest, 
