206 SKELETON. 



In elevation, the head of the os humeri slides downwards in 

 the glenoid cavity, and distends the lower part of the capsular 

 ligament. In this motion the scapula is apt to follow it ; in 

 which case there will be a less degree of distention in the cap- 

 sular ligament. If the os humeri be carried forwards, its ele- 

 vation is performed with much more ease, from the readiness 

 with which the scapula follows it; but if it be carried backwards, 

 this facility is much diminished. It is in the latter position, 

 therefore, that dislocations downwards are most disposed to oc- 

 cur when violence is offered to the joint. If in every case the 

 scapula could follow the motions of the os humeri, so as to pre- 

 sent fairly its glenoid cavity, luxations would be comparatively 

 rare; but generally the violence offered transmits its momentum 

 so speedily to the joint, that the muscles of the scapula are 

 taken by surprise, and have not time to adjust properly the gle- 

 noid cavity. 



In the depression of the os humeri, the parts constituting the 

 shoulder joint are in their most natural and easy position. The 

 capsular ligament becomes very loose below, and is somewhat 

 stretched above. Any degree of force which might be. applied 

 to the member, is warded off and its direction changed by the 

 intervention of the trunk of the body. Should, however, the 

 force be applied directly in the axis of the bone*, the projection 

 of the aeromion process, and the strength of the triangular liga- 

 ment of the scapula, would arrest the dislocation. 



When the os humeri is advanced, the posterior part of the 

 eapsular ligament is put upon the stretch; but the form and ar- 

 rangement of the articular surfaces are somewhat favourable to 

 this position, and accordingly it is one of but little inconve- 

 nience. When the os humeri is retracted, its head, by being 

 directed forwards, exercises considerable force upon the fore 

 part of the' capsular ligament, and when assisted by an exter- 

 nal momentum is disposed to dislocation, forwards and in- 

 wards. 



The motion of circumduction is very extensive in the shoul- 

 der joint; and by it the os humeri describes a cone, of which 

 the glenoid cavity is the apex. It is a regular succession of 

 the movements already mentioned, and in consequence of all 

 the motions forwards of the os humeri being more easy and 



