THE ELBOW-JOINT. 



323 



Long head 



of biceps in 



its tubular 



sheath 



of the 



synovial 



stratum 



Head of 

 humerns 



Cavity of joint 



FIG. 304. VERTICAL SECTION THROUGH THE SHOULDER- JOINT. 



inter tubercular sulcus, yet the closed character of the synovial cavity is maintained. 

 Thus, while the tendon is within the capsule, it is not within the synovial cavity. 

 The synovial stratum is continuous with those bursse which communicate with the 

 joint cavity through openings in the fibrous stratum of the capsule. 



Bursse (a) Communicating with the Joint Cavity. Practically there is only one bursa which 

 is constant in its position, viz., 

 the subscapular, between the 

 capsule and the tendon of the 

 subscapularis muscle. It 

 varies considerably in its 

 dimensions, but its lining mem- 

 brane is always continuous with 

 the synovial stratum of the 

 capsule (Figs. 301 and 302), and 

 therefore it may be regarded 

 merely as a prolongation of 

 the articular synovial stratum. 

 Occasionally a similar but 

 smaller bursa occurs between 

 the capsule and the tendon of 

 the infraspinatus muscle. 



(6) Not communicating with 

 the Joint Cavity. The sub- 

 deltoid or sub-acromial bursa 

 is situated between the muscles 

 on the superior aspect of the 

 shoulder-joint on the one hand 

 and the deltoid muscle on the 

 other. It is an extensive bursa, 

 and is prolonged subjacent to 

 the acromion and the coraco- 

 acromial ligament. It does 

 not communicate with the 

 shoulder -joint, but it greatly facilitates the movements of the proximal end of the humerus 

 against the inferior surface of the coraco-acromial arch. 



Movements at the Shoulder-Joint. A ball-and-socket joint permits of a great variety of 

 movements, practically in all directions ; but if these movements are analysed, it will be seen that 

 they resolve themselves into movements around three primary axes at right angles to each other, 

 or around axes which are the possible combinations of the primary ones. 



Thus, around a transverse axis, the limb may move forwards (flexion) or backwards (extension). 

 Around an antero-posterior axis it may move laterally, i.e. away from the median plane of the 

 trunk (abduction), or medially, i.e. towards, and to some extent up to, the median plane 

 (adduction). 



Around a vertical axis, the humerus may rotate upon its axis in a medial or lateral direction 

 to the extent of a quarter of a circle. 



Since these axes all pass through the shoulder-joint, and since each may present varying 

 degrees of obliquity, it follows that very elaborate combinations are possible until the movement 

 of circumduction is evolved. In this movement the head of the humerus acts as the apex of a 

 cone of movement with the distal end of the humerus, describing the base of the cone. 



The range of the shoulder-joint movements is still further increased owing to the mobility of 

 the scapula as a whole, and owing to its association with the movements of the clavicle already 

 i described. 



ARTICULATIO CUBITI. 



The elbow-joint 1 provides an instance of a diarthrosis capable of performing the 

 ! movements of flexion and extension around a single axis placed transversely, i.e. a 

 typical ginglymus diarthrosis or hinge-joint. 



The bones which enter into its formation are the humerus, ulna, and radius. 

 ; The trochlea of the humerus articulates with the semilunar notch of the ulna 

 (articulatio humeroulnaris) ; the capitulum of the humerus articulates with the 

 shallow depression or cup on the proximal aspect of the head of the radius (articu- 

 latio humeroradialis). The articular cartilage clothing the trochlea of the humerus 

 'terminates in a sinuous or concave margin both anteriorly and posteriorly, so that it 

 loes not line either the coronoid or the olecranon fossa. Medially, it merely rounds 

 off the medial margin of the trochlea, but laterally it is continuous with the encrust- 



1 The articulatio cubiti or elbow-joint includes the humero-radial, humero-ulnar, and the proximal radio-ulnar 

 oints ; but, for convenience, the description given here is limited to the humero-radial and humero-ulnar joints. 



