It lies (1) in the intimate manner in which the scapular muscles are arranged around it; (2) in the overhanging coraco-acromial arch which forms, as it were, a secondary socket for the head of the humerus, and effectually prevents any displacement in an upward direction; and (3) in atmospheric pressure, which exercises a powerful influence in keeping the opposed surfaces in contact with each other.

From all points of view, except over a small area below, the loose, ligamentous capsule which envelops the shoulder joint is supported by muscles, the tendons of which are more or less intimately connected with it. Above, it is covered by the supraspinatus; behind, the infraspinatus and teres minor are applied to it; in front is the subscapularis. Below, the capsule is to a certain extent unsupported by muscles, and here it is prolonged downwards, in the form of a fold, in the ordinary easy dependent position of the limb (Fig. 24). When, however, the arm is abducted, this fold is obliterated, and the head of the bone rests upon the inferior part of the capsule, which now receives partial support from two muscles which are stretched under it, viz., the long head of the triceps and the teres major. Still, this must be regarded as the weakest part of the joint, and consequently dislocation of the head of the humerus, downwards into the axilla through the inferior part of the capsule, is an occurrence of considerable frequency.

Dissection.-Detach the axillary vessels and brachial nerves from the coracoid process to which they have been tied, and throw them downwards. Then proceed to remove the muscles. Divide the conjoined origin of the short head of the biceps and the coracobrachialis close to the coracoid process, the teres major about its middle, and the long head of the triceps about an inch or two below its origin, and turn them aside. Next deal with the muscles more immediately in relation to the joint, viz., the supraspinatus, the infraspinatus, the teres minor, and the subscapularis. These must be removed with great care and deliberation, because their tendons are closely connected with the subjacent ligamentous capsule. They are not incorporated with the capsule, however, although at first sight they appear to be so, and thus they can be dissected from it. In the case of the subscapularis a protrusion of the synovial membrane, forming a bursa, will be found near its upper border, close to the root of the coracoid process. The capsule of the shoulder-joint may now be cleaned, and its attachments defined.

The Ligaments in connection with the shoulder-joint

The Capsular Ligament (capsula articularis) is a dense and strong ligamentous structure, which envelops the shoulder-joint on all sides. It is attached to the scapula around the glenoid cavity, but only above is it directly fixed to the bone. Elsewhere it springs from the fibrous ring or glenoid ligament, which serves to deepen the articular cavity; indeed, in its lower part, it appears to be nearly continuous with the border of the glenoid ligament. Externally it is fixed to the outer part of the anatomical neck of the humerus. The width of the capsule is not uniform throughout. It will be noticed to expand as it passes over the enclosed head of the humerus, and to contract as it reaches its scapular and humeral attachments. The great laxity of the capsule of the shoulder-joint will now be apparent. When the muscles are removed, and air is admitted into the joint, the bony surfaces fall away from each other-the head of the humerus sinking downwards, when the part is held by the scapula, to the extent of an inch.

The capsule of the shoulder-joint is not complete upon all aspects. Its continuity is interrupted by two, and sometimes three, apertures. The largest of these is an opening of some size, which is placed upon its inner or anterior aspect, near the root of the coracoid process. Through this aperture an extensive protrusion of the synovial membrane takes place in the form of a synovial bursa, which, from its position under the upper part of the subscapularis muscle, receives the name of the bursa subscapularis. It is important to note the position and character of this opening, seeing that in some cases the head of the bone may be driven through it in dislocation of the joint. The second aperture is smaller and more distinctly defined. It is placed between the two tuberosities of the humerus, at the upper part of the bicipital groove, and it is through this that the long tendinous head of the biceps gains. admission to the interior of the capsule. The synovial membrane also protrudes from this opening, and lines the bicipital groove as low as the insertion of the pectoralis

major. It is not often that the third opening is seen.

It is situated, when present, on the outer or posterior aspect of the capsule, and allows a pocket of synovial membrane to bulge out in the form of a bursa under the infraspinatus muscle.

At certain points the capsule of the shoulder-joint is specially thickened by the addition of fibres, which pass from the scapula to the humerus. Two of these thickened portions receive the names of the coraco-humeral and the gleno-humeral ligaments. A third is placed on the inferior aspect of the capsule, where it is not supported by muscles, viz., between the long head of the triceps and the subscapularis muscles. It is against this thickened portion of the capsule that the head of the humerus rests when the arm is abducted from the side, and it is sometimes spoken of as the inferior accessory ligament, or inferior gleno-humeral ligament.

The Coraco-humeral Ligament (ligamentum coracohumerale) is placed upon the upper aspect of the joint. It is a broad band of great strength, which is more or less completely incorporated with the capsule. Above, it

is fixed to the root and outer border of the coracoid process of the scapula, and it passes from this obliquely downwards and outwards, to gain attachment to the two tuberosities of the humerus. It forms a strong arch over the upper part of the bicipital groove, under which the tendon of the biceps passes.

The Gleno-humeral Ligament can only be seen when the joint is opened. The dissector should therefore, at this stage, remove the posterior part of the capsule, and, drawing the bones well apart from each other, look forwards into the cavity. The tendon of the biceps will be observed arching over the head of the humerus, to reach its insertion on the upper aspect of the glenoid cavity. Immediately internal to this, and parallel to it, will be noticed a ridge on the inner aspect of the capsule projecting into the joint. This band is the gleno-humeral ligament (of Mr. Flood). It is inserted into a faintly-marked pit on the anatomical

neck of the humerus, close to the upper end of the bicipital groove.

Dissection.-Complete the division of the capsular ligament, and drawing the tendon of the biceps through the intertubercular aperture in the capsule, separate the two bones from each other.

Glenoid Ligament (labrum glenoidale).-The glenoid ligament is the dense fibro-cartilaginous band which surrounds the margin of the glenoid cavity of the scapula, and is attached to its rim. It deepens, and at the same time serves to extend, the articular socket of the scapula. The intimate connection which it presents with the capsule of the joint can now be studied. Two tendons are also closely associated with it, viz., the long head of the triceps below, and the long head of the biceps above.

The Long Head of the Biceps is an important factor in the construction of the shoulder-joint. Entering the capsule through the opening between the two tuberosities of the humerus, it is prolonged over the head of the bone to the top of the glenoid cavity. Its insertion at this point should now be examined. It will be seen to divide into three portions, viz., a large intermediate part, which obtains direct attachment to the scapula, and two smaller lateral parts, which diverge from each other and blend with the glenoid ligament. The long head of the biceps, by its position within the capsule, and in the deep groove between the tuberosities of the humerus, serves to keep the head of the bone in place, and to steady it in the various movements at the shoulder-joint.

Synovial Membrane.-The synovial membrane lines the interior of the capsular ligament, and is reflected from it upon the anatomical neck of the humerus as far as the articular margin of the head of the bone. The bursal protrusion of the synovial membrane (bursa subscapularis) under the tendon of the subscapularis muscle has already been noticed. The tendon of the biceps, as it traverses the joint, is enveloped in a tubular sheath of the membrane,

which bulges out through the opening of the capsule in the form of a bursa, which lines the bicipital groove, and receives the name of bursa intertubercularis.

The Articular Surfaces.-The smooth, glistening articular cartilage, which coats the head of the humerus, is thickest in the centre, and thins as it passes towards the edges. In the case of the glenoid cavity the reverse of this will be noticed. The cartilaginous coating is thinnest in the centre, and becomes thicker as it is traced towards the circumference.

Movements at the Shoulder-joint.-The shoulder is a ball and socket joint, and consequently movement in every direction is permitted, viz.-(1) flexion, or forward movement; (2) extension, or backward movement (checked in its extent by the coraco-humeral ligament); (3) abduction, or outward movement (checked by the coraco-acromial arch); (4) adduction, or inward movement (limited by the coraco-humeral ligament). In addition to these different forms of angular movement, rotation to the extent of a quarter of a circle and circumduction are permitted.

The muscles chiefly concerned in producing these movements areflexion-the pectoralis major and the anterior part of the deltoid; extension-latissimus dorsi, posterior part of the deltoid, and the teres major; abduction—the deltoid and supraspinatus; adduction—pectoralis major, coraco-brachialis, teres major, and latissimus dorsi; rotation inwards—subscapularis, pectoralis major, latissimus dorsi, teres major ; rotation outwards-supraspinatus, infraspinatus, and teres minor; circumduction is produced by the action of different combinations of these muscles.

FOREARM AND HAND.

Dissection. The skin has already been removed from the front and back of the forearm. It should now be raised from the dorsum of the hand by making incisions along the radial and ulnar borders. This is done in order that the superficial structures in this region may be examined in connection with those of the forearm.

Superficial Veins.—On the dorsum of the hand a plexus of superficial veins will be seen. In defining this, care must be taken of the fine cutaneous twigs from the radial nerve and the dorsal branch of the ulnar nerve. From the