SHOULDER JOINT (NORMAL ANATOMY). 



573 



occasion to point out the abundant provisions 

 which exist to counteract this tendency. 



As regards its motions, and the anatomical 

 dispositions of its connecting media, the 

 shoulder joint belongs to the class of " Enar- 

 throdial Articulations ;" but, if its bony con- 

 stituents alone be considered, it seems more 

 nearly allied to the " Arthrodia." Tin's is 

 owing to the imperfect development of the 

 glenoid cavity which is opposed to the head 

 of the hum ems. 



The shoulder joint is constructed after the 

 same plan in all vertebrate animals whose 

 anterior extremities are developed. 



In this article the several components of 

 the scapulo-humeral articulation shall be de- 

 scribed in the following order: 



I. THE BONES. 2. THE STRUCTURES 



\VHICH FACILITATE THEIR MOTIONS. O. Arti- 

 cular Fibro-Cartilage. b. Articular Cartilage. 

 c. Synovial Membrane, 3. THE CONNECTING 

 MEDIA. a. Passive connecting Media the 

 Ligaments, b. Active connecting Media the 

 Muscles ; in connection with the detail of the 

 Mechanical Functions of the joint. 



1. Bones. We shall speak of these briefly, 

 as they have been already described in the 

 article Extremity. 



The bones which enter into the formation 

 of the shoulder joint are, the head of the hu- 

 merus and the glenoid cavity of the scapula. 

 These opposed surfaces are of very dispro- 

 portionate size, the shallow cavity of the sca- 

 pula not exceeding in dimensions one-third of 

 the head of the humerus. 



The glenoid cavity is placed at the anterior 

 superior angle of the scapula, below and be- 

 tween the acromion and coracoid processes ; 

 a slight constriction, the neck of the scapula, 

 separates it, together with the coracoid pro- 

 cess, from the body of the scapula ; superiorly, 

 the neck of the scapula traverses the notch in 

 the superior costa of the bone, behind the 

 base of the coracoid process ; infcriorly, it 

 terminates close to the lower extremity of the 

 articular surface. The aspect of the glenoid 

 cavity in the quiescent state of the scapula, is 

 upwards, forwards, and outwards ; it presents 

 an ovoid outline, the larger end below, and 

 the smaller above. A vertical line falling 

 upon the axillary margin of the scapula di- 

 vides this articular cavity into two unequal 

 portions, of which the inner is the larger. 

 This arrangement in some degree diminishes 

 the tendency to displacement inwards of the 

 head of the humerus, to which, for other 

 reasons, the joint is strongly disposed. 



An "arrest of development" may cause a 

 deficiency of either the outer or the inner lip 

 of the glenoid cavity, resulting in a congenital 

 dislocation of the head of the humerus, in- 

 wards or outwards, according to the portion 

 of the cavity which is deficient. These con- 

 stitute the "sub-acromial" and " sub-cora- 

 coid " dislocations described by Dr. 11. Smith.* 

 Boyer supposes that a deficient develop- 



* Dublin Journal of Medical Science, vol. xv., 

 1839. 



ment of the outer lip of the glenoid cavity 

 must pre-exist, in order to permit the dislo- 

 cation backwards on the dorsum of the sca- 

 pula to occur.* 



A little external to its apex, a slight notch 

 in the margin of the glenoid cavity marks the 

 place of attachment of the long tendon of the 

 biceps; whilst on its upper and inner side a 

 shallow groove points out the passage of the 

 tendon of the subscapularis muscle. 



The head of the humerus presents a convex 

 hemispherical surface, the aspect of which is 

 upwards, backwards, and inwards. An irre- 

 gular wavy line separates the head from the 

 anatomical neck of the bone, the latter inter- 

 vening between the head and the tuberosities. 

 The line which marks the union of the upper 

 epiphysis with the shaft of the humerus has 

 been long incorrectly described, as though it 

 were identical with the anatomical neck. The 

 upper epiphysis comprises not only the head 

 of the bone, but also the tube rositie* ; for 

 though, doubtless, the line of junction between 

 the upper epiphysis and the shaft corresponds 

 internally to the anatomical neck immediately 

 beneath the cartilage of incrustation, yet from 

 this its direction is chiefly outwards, so that 

 externally it passes below the greater and the 

 lesser tuberosities, traversing the bicipital 

 groove which is included between them. 



This anatomical fact, and the practical in- 

 ferences derivable from it, have been clearly 

 pointed out by Dr. R. Smith.f 



2. Structures ivhich facilitate motion in the 

 joint. a. The border of the glenoid cavity 

 has attached to it a fibro-cartilaginous rim 

 {glenoid ligament) by which the depth of the 

 cavity is somewhat increased. This structure 

 is thickest at its attachment to the bone ; its 

 free edge is very thin ; a section of it made at 

 right angles to the bone gives it a triangular 

 outline. Both its surfaces are lined by sy- 

 novial membrane, which consequently sepa- 

 rates it externally from the capsuiar ligament ; 

 superiorly, many fibres of the biceps tendon 

 become continuous with the fibrous portion 

 of the so-called ''glenoid ligament," and after 

 prolonged maceration the tendon will separate 

 from the bone along with this structure, but 

 to describe the glenoid ligament as formed 

 by the splitting of the tendon of the biceps, 

 would be erroneous. The glenoid ligament 

 is subservient to the following purposes: it 

 deepens the shallow glenoid cavity, and so 

 lessens the liability to dislocation ; it prevents 

 the bony surfaces of the neck of the humerus, 

 and the edge of the glenoid cavity, from being 

 unduly pressed against each other in the ex- 

 tensive motions of the joint; and it gives a 

 more extended, and therelbre a more secure 

 attachment to the tendon of the biceps. 



b. The articular surfaces are invested with 

 cartilage of incrustation, which, in accordance 

 with a very general rule, is much thicker at 

 the centre of the convex head of the humerus 



' Traile' des Maladies Chirurgicales, torn. iv. p. 

 1 7(1. 



t Essay on Fractures, &c., [>. L'03. Dublin, 184S. 



