THE ARTICULATIONS OF THE BONES OF THE FOREARM. 169 



Movements. The shoulder-joint, being 1 a ball and socket joint, allows of angular move- 

 ment in all its varieties, as well as of rotation. The freedom of movement in different 

 directions is determined by the extent of the humeral articular surface and the length of the 

 capsule. In extreme positions the neck of the humerus meets the fibrous margin of the 

 glenoid cavity on the side towards which the movement takes place, and the capsule is 

 stretched on the opposite side of the joint. In flexion the humerus is carried forwards and 

 inwards, in extension backwards and outwards, the head of the bone revolving upon an axis 

 which is nearly perpendicular to the centre of the glenoid cavity ; in abduction and adduction 

 the arm moves in directions at right angles to the foregoing, the axis being horizontal and 

 parallel to the surface of the glenoid fossa. The range of movement of the humerus upon 

 the scapula in passing from extreme adduction to extreme abduction is but little more than 

 90, and in the direction of flexion and extension is still less. The greater freedom of move- 

 ment enjoyed by the arm, which can be raised from the body until it is directed nearly 

 vertically upwards, is due to rotation of the scapula (p. 165), which always accompanies these 

 movements of the shoulder-joint, the glenoid cavity being turned upwards when the arm is 

 elevated, and sinking again as the limb is depressed. In rotation the humerus revolves 

 about its long axis ; the whole range of the movement is about a quarter of a circle. The 

 arch formed by the acromion, the coracoid process, and the coraco-acromial ligament, lined by 

 the subacromial bursa, forms a sort ^pf secondary socket in which the extremity of the 

 humerus, covered by the tendons inserted into the great tuberosity, revolves, and against 

 which it is pressed when the weight of the body is made to rest upon the arms. (C. W t 

 Cathcart, Journ. Anat., xviii, 211 ; Cleland, ib., 275.) 



THE ARTICULATIONS OF THE BONES OF THE FOREARM. 



The bones of the forearm are united by a superior and an inferior articulation 

 and an interosseous membrane. 



In the SUPERIOR RADIO-ULNAR ARTICULATION the head of the radius is received 

 into the small sigmoid cavity of the ulna and is held in position by the annular 

 or orbicular ligament. This ligament is a strong band of fibres attached to the 



Fig. 196. THE UPPER PART OP THE ULNA, WITH THE ORBICULAR 

 LIGAMENT. (R. Quain.) 



1, upper division of the sigmoid surface on the olecranon ; 2, extremity of 

 the coronoid process ; 5, orbicular ligament. 



ulna in front and behind, at the extremities of the small 2 

 sigmoid cavity, and forming about four-fifths of a ring which 

 encircles the head of the radius and binds it firmly in its situa- 

 tion. The external lateral ligament of the elbow is inserted 

 into its outer surface ; its deep surface is smooth, and is lined 

 by the synovial membrane of the elbow joint. 



INFERIOR RADIO-ULNAR ARTICULATION. The connection 

 between the sigmoid cavity of the radius and the lower end 

 of the ulna is effected by means of a fibro-cartilage, a synovial 

 sac, and some scattered ligamentous fibres in front and behind. 

 The triangular fibro-cartilage is a thick plate attached by its base to the border 

 separating the carpal from the ulnar articulating surface of the radius ; and by its apex 

 to a depression at the root of the styloid process of the ulna, and to the outer side of 

 that process. Its upper surface looks towards the ulna, its lower towards the lunar 

 bone, and it separates the inferior radio-ulnar articulation from the wrist-joint. The 

 synovial sac, which is very loose, extends upwards between the radius and ulna, 

 and horizontally inwards between the ulna and triangular fibro-cartilage. When 

 the fibro-cartilage is perforated, as is frequently the case, this synovial cavity com- 

 municates with that of the wrist-joint. 



The interosseous membrane or ligament of the forearm is a strong fibrous 

 membrane, the fibres of which are directed for the most part obliquely downwards and 



