350 HEALTH AND DISEASE 



SHOULDER SLIP 



By this term is understood a loosening of the union between the 

 bones forming the shoulder-joint, in consequence of which the head of 

 the humerus or arm-bone is outwardly displaced from the shallow cavity 

 into which it fits on the lower end of the scapula or blade-bone. This 

 disease is invariably associated with paralysis of the muscles of the 

 shoulder, which sooner or later waste, and by ceasing to give support 

 to the joint allow the bones to become displaced, and the limb as an 

 organ of support to be weakened and deformed. It is more especially 

 seen in young horses engaged in agricultural work, although adults are 

 by no means exempt from it. 



Causes. The precise origin of the disease is but imperfectly under- 

 stood. That the muscles of the shoulder are paralysed is clear enough, but 

 as to how that paralysis is brought about there is considerable diversity of 

 opinion. By some it is regarded as the result of an injury to the supra- 

 scapular nerve by blows, and over-stretching in the performance of certain 

 backward movements of the shoulder and limb, involving great strain. By 

 others it is said to arise out of inflammation of the muscles of the shoulder, 

 induced by the stretching they undergo when slipping in and out of the 

 furrow in the act of ploughing, and in habitually walking on an irregular 

 surface. Of the two alleged causes our view is in favour of the former. 

 We cannot understand shoulder-slip being of such rare occurrence in face 

 of the very large number of young horses which are every day at plough, 

 following the occupation which is said to induce it. 



Symptoms. These vary somewhat in different cases as far as refers 

 to the onset of the disease. In some there are no clearly-defined local 

 symptoms until the muscles have become noticeably atrophied, and as they 

 waste the shoulder is seen to shrink, leaving the spine in the centre of the 

 blade-bone standing out prominently as a sharp projecting ridge with a 

 hollow on either side. At the same time the point of the shoulder bulges 

 outward, and the foot of the affected limb is drawn inward towards the 

 other one, so that the leg, instead of following a straight line from the 

 shoulder to the ground, takes a slanting direction inwards. In respect to 

 locomotion, the main features of these cases are a disposition to trail the 

 toe, inability to bring the limb well forward, and the outward movement 

 which it describes when an attempt is made to do so. When the weight of 

 the body is placed upon it, the shoulder is forced outward, and there is a 

 perceptible descent of the quarter on the affected side. 



These symptoms may be preceded by more or less heat and swelling in 



