144: THE ARTICULATIONS. 



there is the atmospheric pressure, whose influence is of a certain impor- 

 tance. This may be proved by removing all the surrounding muscles, when 

 it will be found that the capsule is not relaxed, nor are the articular surface* 

 separated ; to effect this, it is necessary to make an opening in the capsule, 

 so as to allow the air to enter its cavity, when the surfaces immediately 

 separate. 



Synovial capsule. This is very loose, and entirely enveloped by the 

 peripheral capsule, whose internal surface it lines. 



Movements. Like all the enarthrodial articulations, the scapulo-humeral 

 permits extension, flexion, abduction, adduction, circumduction, and rotation. 

 These various movements, however, are far from being so extensive as in 

 Man, the arm in the domesticated animals not being detached from the 

 trunk, but bsiiig, on the contrary, fixed with the shoulder against the lateral 

 parietes of the thorax. Flexion and extension are the least limited and the 

 most frequently repeated movements; their execution always demands a 

 displacement of the two bones, which are almost equally movable. Inflexion^ 

 the scapulo-humeral angle is closed, not only because the inferior extremity 

 of the huinerus is carried backwards and upwards, but also because the 

 scapula pivots on its superior attachments in such a manner as to throw its 

 glenoid angle forward and upward. Extension is produced by an inverse 

 mechanism. During the execution of the other movements, the scapula, 

 remains fixed, and the humerus alone is displaced, bringing with it the 

 inferior rays of the limbs. If it is carried outwards, we have abduction, 

 or inwards, adduction ; if the member passes successively from flexion to 

 abduction, and from that to extension, etc., in describing a circle by its 

 lower extremity, then there is circumduction; if it pivots from left to right, 

 or right to left, we have rotation. 



In the Pig, Dog, and Cat, the synovial membrane is not exactly inclosed by the 

 fibrous capsule, but forms in front a cul-de-sac, which descends in the bicipital groove to 

 favour the gliding of the coraco-radial tendon. 



In Man, the scipulo-humeral articulation is disposed as in animals, but it is also 

 protected above by the coraco -acromion 'roof. For the reasons noted above, this 

 articulat'on allows of more extensive motion than in animals. As remarked by 

 Cruveilhier, of all the joints in the human body, the scapulo-humeral is that which has 

 the most extensive motion; in movements forward and outward, the humerus can 

 become horizontal; in those of c ire umd action it describes a complete cone, which is 

 more extensive in front and laterally than behind and inwardly. 



2. Humero-radial, or Elbow Articulation. 



(Preparation. Turn down the inferior extremity of the flexors of the fore-arm, 

 remove the olecranian, epicondyloid, and epitrochlean muscles, taking care not to 

 damage the ligaments to which they somewhat closely adhere.) 



Three bones concur to form this articulation, which presents a remark- 

 able example of an angular ginglymus : the humerus, by its inferior ex- 

 tramity, and the two bones of the arm by their upper extremities. 



Articular surface*. The humeral surface, already described at page 74, 

 is transversely elongated, and convex from before to behind. It presents : 

 1, A median groove excavated by a synovial fossette ; 2, An external groove 

 (numeral trochlea) not so deep as the preceding ; 3. A kind of voluminous 

 condyle which borders, inwardly, the internal pulley, and whose antero- 

 posterior diameter is much greater than that of the external lip of the 

 trochlea of the opposite side. The antibrachial surface, divided into two 

 portions, is moulded to the humeral surface ; it is, therefore, concave before 

 and behind, and is composed : 1, Of a double external groove ; 2, Of an 



