138 ARTICULATIONS OF THE UPPER LIMB. 



its origin, where it again becomes continuous with the synovial membrane 

 of the capsule in such a manner as to preserve the integrity of the mem- 

 brane. The bursal prolongation of the synovial membrane on the tendon of 

 the subscapularis muscle is of variable extent, sometimes scarcely existing, 

 sometimes forming a considerable pouch on the venter of the scapula. 



Subacromial Bursa. Superficial to the muscles covering the top of the 

 joint is a considerable bursa mucosa, by means of which the contiguous sur- 

 faces of the coracoid and acromion processes, and of the coraco-acromial 

 ligament and deltoid muscle, are lubricated, so as to facilitate the movements 

 of the subjacent head of the humerus. 



MOVEMENTS. Great freedom of movement of the humerus in every direction is 

 admitted at the shoulder-joint ; but superiorly and posteriorly the extent of the move- 

 ment is somewhat limited by the margin of the acromion. When the arm is raised, 

 the great tuberosity of the humerus becomes locked against the acromion as soon as the 

 position is reached in which the limb lies at right angles to the trunk, and all further 

 elevation is accomplished by movements in the sterno-clavicular and acromio-clavicular 

 articulations. The arch formed by the acromion, the coracoid process, and the deltoid 

 ligament, lined by the subacromial bursa, forms a sort of secondary socket, in which 

 the extremity of the humerus, covered by the tendons inserted into the great tube- 

 rosity, revolves, and against which it is pressed when the weight of the body is made 

 to rest upon the arms : in quadrupeds, the weight of the body is transmitted through 

 the glenoid fossa to the humerus. 



ARTICULATIONS OF THE FOREARM AND ELBOW. 



UNION OF THE RADIUS AND ULNA. The bones of the forearm are united 

 by a superior and an inferior articulation and an interosseous membrane. 



Fig. 127. THE UPPER PART OP THE ULNA, WITH THE 

 ORBICULAR LIGAMENT OP THE RADIUS. ^ 



] , upper division of the sigmoid surface on the olecranon ; 

 2, extremity of the coronoid process ; 5, orbicular liga- 

 ment. 



In the superior radio-ulnar articulation, the con- 

 nection of the head of the radius with the small sig- 

 moid cavity of the ulna is effected by means of the 

 annular or orbicular ligament, a strong baud of fibres 

 attached to the ulna in front and behind at the ex- 

 tremities of the small sigmoid cavity, and forming 

 four- fifths of a ring which encircles the head of the 

 radius and binds it firmly in its situation. The outer 

 surface is connected with the external lateral ligament 

 of the elbow, whose fibres are inserted into it ; the 

 deep surface is smooth, and is lined by the synovial 

 membrane of the elbow-joint. 



The inferior radio-ulnar articulation. The connection between the semi- 

 lunar surface of the radius and the lower end of the ulna is effected by 

 means of a fibro-cartilage, a synovial membrane, and some scattered liga- 

 mentous fibres in front and behind. The triangular fibro-cartilage is a thick 

 plate attached by its base to a ridge 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 side of that process. Its 

 upper surface looks towards the ulna, its lower towards the cuneiform bone, 



