SHOULDER-SLIP. 257 



perform the function of ligaments, and admit of a variety 

 and extent of motion "by their power of contraction and re- 

 laxation. 



These muscles are the antea and postea spinatxLs and teres ex- 

 temus. The spinati, which are mostly concerned in this, are 

 lodged in the fossa of the scapula, covering its whole external 

 surface, and attached inferiorly to the tuberosities and ridge of 

 the hvimerus, and to the capsxilar ligament of the shoulder- 

 joint. It will be seen from their position that their action is 

 mainly to prevent the outward bvdging of the head of the 

 humerus, to keep the parts firmly and closely in their respec- 

 tive situations, and to assist in, the elevation and adduction of 

 the humerus. 



It will be rememhered that there is great disparity between 

 the articular head of the humerus and the glenoid cavity of 

 the shoulder, the latter being much smaller than the former. 

 This disparity of size allows a liberty of motion without dis- 

 location that otherwise would be impossible, assisted and 

 kept within moderate bounds hy the action of the /oregoing 

 muscles. 



This form of lameness is generally seen in horses that are 

 worked in the plough, and in the horse that works in the 

 furrow. Mr. Barlow said, " always in the limb that is placed 

 in the furrow ;" but I cannot endorse this, having often seen it 

 in both shoulders. 



The explanation is, I think, simple, the functions of the 

 muscles being — Ist. To keep the parts firmly together; 2d. To 

 prevent the outward bulging of the head of the humerus ; 3c?. 

 To draw the head of humerus outwards when the glenoid cavity 

 overlaps its outer border ; and Ath, To assist in lifting the limb. 

 Now, when the limb is, as it were, made longer than its fellov?', 

 by the foot being placed upon lower ground in the furrow, it 

 necessarily follows that the work to be performed by these 

 muscles is increased to a considerable extent, as the limb seems 

 to hang from the shoulder ; and that, in consequence, the demand 

 "upon them is beyond their power of endvirance. Inflammation 

 is excited in the muscular structure, and often in the tendons ; 

 the exudation presses upon the true sarcons fibrillse, which, along 

 with degradation of their constituent elements consequent upon 

 the injury, leads on to atrophy. 



8 



