THE SHOULDER-JOINT. 251 



angle with the trunk, the further elevation of the limb being effected by the 

 Trapezius and Serratus magnus moving the scapula on the wall of the chest. 

 This mobility is of special importance in ankylosis of the shoulder-joint, the 

 movements of this bone compensating to a very great extent for the immobility 

 of the joint. 



IV. Shoulder-joint (Fig. 169). 



The Shoulder is an enarthrodial or ball-and-socket joint. The bones entering 

 into its formation are the large globular head of the humerus, which is received 

 into the shallow glenoid cavity of the scapula an arrangement which permits of 

 very considerable movement, whilst the joint itself is protected against displacement 

 by the tendons which surround it and by atmospheric pressure. The ligaments do 

 not maintain the joint surfaces in apposition, because when they alone remain the 

 humerus can be separated to a considerable extent from the glenoid cavity ; their 

 use, therefore, is to limit the amount of movement. Above, the joint is protected 

 by an arched vault, formed by the under surface of the coracoid and acromion 

 processes, and the coraco-acromial ligament. The articular surfaces are covered by 

 a layer of cartilage : that on the head of the humerus is thicker at the centre than 

 at the circumference, the reverse being the case in the glenoid cavity. The liga- 

 ments of the shoulder are the 



Capsular. Transverse humeral. 



Coraco-humeral. Glenoid. 1 



The Capsular Ligament completely encircles the articulation, being attached, 

 above, to the circumference of the glenoid cavity beyond the glenoid ligament ; 

 below, to the anatomical neck of the humerus, approaching nearer to the articular- 

 cartilage above than in the rest of its extent. It is thicker above and below than 

 elsewhere, and is remarkably loose and lax, and much larger and longer than is 

 necessary to keep the bones in contact, allowing them to be separated from each 

 other more than an inch an evident provision for that extreme freedom of move- 

 ment which is peculiar to this articulation. Its superficial surface is strengthened, 

 above, by the Supraspinatus ; below, by the long head of the Triceps ; behind, by 

 the tendons of the Infraspinatus and Teres minor ; and in front, by the tendon of 

 the Subscapularis. The capsular ligament usually presents three openings ; one 

 anteriorly, below the coracoid process, establishes a communication between the 

 synovial membrane of the joint and a bursa beneath the tendon of the Subscapu- 

 laris muscle. The second, which is not constant, is at the posterior part, where a 

 communication sometimes exists between the joint and a bursal sac belonging to 

 the Infraspinatus muscle. The third is seen between the two tuberosities, for the 

 passage of the long tendon of the Biceps muscle. 



The Coraco-humeral is a broad band which strengthens the upper part of the 

 capsular ligament. It arises from the outer border of the coracoid process, and 

 passes obliquely downward and outward to the front of the great tuberosity of the 

 humerus, being blended with the tendon of the Supraspinatus muscle. This liga- 

 ment is intimately united to the capsular in the greater part of its extent. 



Supplemental Bands of the Capsular Ligament. In addition to the coraco- 

 humeral ligament, the capsular ligament is strengthened by supplemental bands in 

 the interior of the joint. One of these bands is situated on the inner side of the 

 joint, and passes from the inner edge of the glenoid cavity to the lower part of the 

 lesser tuberosity of the humerus. This is sometimes known as Flood's ligament, 

 and is supposed to correspond with the ligamentum teres of the hip-joint. A 

 second of these bands is situated at the lower part of the joint, and passes from the 

 under edge of the glenoid cavity to the under part of the neck of the humerus, and 

 is known as Scldemms ligament. A third, called the c/leno-humeral ligament, is 

 situated at the upper part of the joint, arid projects into its interior, so that it can 



1 The long tendon of origin of the Biceps muscle also acts as one of the ligaments of this joint. 

 See the observations on p. 222 on the function of the muscles passing over more than one joint. 



