ABTICVLATIOm OF THE ANTEBIOB LIMBS. 143 



spine and sternum, it is advantageous to study the mechanism which presides 

 in the execution of their movements. 



The costal arches being inclined backwards on the middle plane the 

 space they inclose in their concavity is not nearly so extensive as if they 

 had been perpendicular to this plane. Owing to their double arthrodial 

 joints, the ribs are movable on the spine, and their inferior extremity, also 

 movable, rests either directly or indirectly on the sternum. Therefore it is 

 that, when they are drawn forward by their middle portions, they pivot on 

 their extremities, and tend to assume a perpendicular direction, which is the 

 most favourable for the largest iacrease of the space they limit ; then there 

 is enlargement of the lateral diameter of the thorax, which signifies dilatation 

 of its cavity. The inverse movement, by an opposite mechanism, causes 

 the contraction of the chest. 



The ribs are said to be elevated during the forward movement, and 

 d&pressed when they fall backwards. These expressions, though perfectly 

 applicable to Man, who stands in a vertical position, are not correct when 

 employed in veterinary anatomy. 



Article IV. — Articitlations of the Anterioe, Limbs. 



1. Scwpulo-humeral Articulation. 



{Preparation. — ^Detach the limb from the trunk. Kemove from the upper extremity 

 those muscles -whioh are inserted in the vicinity of the glenoid cavity of the scapula ; 

 turn down from its lower extremity those which are inserted into the superior end of 

 the humerus or a little below, preserving the attachments of their tendons with the 

 capsular ligament. The thin scapulo-humeralis muscle may be. allowed to remain in 

 order to show its relations.) 



To constitute this enarthrodial articulation, the scapula is united to the 

 humerus, and forms an obtuse angle which is open behind. 



Articular surfaces. — In the scapula there is the glenoid cavity, the 

 shallow, oval fossa, elongated in an antero-posterior direction, notched in- 

 wardly, and excavated at its centre or near the internal notch by a small synovial 

 fossette. A ligamentous band, attached to the brim of the cavity, fills up 

 this notch, and is the vestige of the glenoid ligament of man. In the 

 humerus, the articular head, fixed between the large and small tuberosities, 

 is often excavated by a shallow synovial fossette. 



Mode of union. — One capsular ligament (Fig. 85, 1), a kind of sac having 

 two openings : one inferior, embracing the head of the humerus ; the 

 superior, inserted into the margin of the glenoid cavity. This capsule pre- 

 sents in front two supporting fasciculi, which diverge as they descend from 

 the coracoid process to the great and small tuberosities. The aponeurotic 

 expansion thus formed is very thin and loose, so as to allow the two bones to 

 separate to the extent of from -j*,- to -^ of an inch ; but it is far from 

 being sufficiently strong to bind them firmly together. The ai tioulation is, 

 therefore, consolidated by the powerful muscles which surround it, among 

 which may be noticed: 1, In front, the coraco-radial (flexor brachiij, 

 separated from the fibrous capsule by an adipose cushion ; 2, Behind, the 

 large extensor of the fore-arm and thin scapulo-humeral (teres minor) 

 muscles, whose office appears to be to pull up this capsule during the move- 

 ments of flexion, so as to prevent its being pinched between the articular 

 surfaces ; 3, Outwards, the short abductor of the arm and the subspinous 

 (postea spinatus) tendon ; 4, Inwards, the wide and strong tendon of the 

 subscapular muscle. In addition to these powerful retaining apparatus. 



