long or outer head (caput longum) is a rounded tendon, which occupies the bicipital groove of the humerus. Its origin cannot be studied at this stage of the dissection, because it is placed within the capsule of the shoulder-joint. Suffice it for the present to say, that it arises from an impression on the scapula immediately above the glenoid fossa. Both heads swell out into elongated fleshy bellies, which are closely applied to each other, and afterwards unite in the lower third of the arm. Towards the bend of the elbow the fleshy fibres converge upon a stout, short tendon, which is inserted into the posterior part of the tuberosity of the radius. This insertion will be more fully examined at a later period, but it may be noticed in the meantime that the tendon is twisted so as to present its margins to the front and back of the limb, and further, that a synovial bursa is interposed between it and the anterior smooth part of the radial tuberosity.

The dissector has already taken notice of the bicipital or semilunar fascia, and has separated it artificially from the brachial aponeurosis above, and from the deep fascia of the forearm below. Observe now that it springs from the anterior margin of the tendon of the biceps, and that it likewise receives some muscular fibres from the short head of the muscle.

The Brachialis Anticus (musculus brachialis) arises from the entire width of the anterior surface of the lower half of the shaft of the humerus, from the internal intermuscular septum, and from a small part of the external intermuscular septum above the supinator longus. The origin from the bone is prolonged upwards in two slips which partially embrace the insertion of the deltoid. The fibres converge to be inserted into the base of the coronoid process of the ulna by a short, thick tendon. The muscle lies partly under cover of the biceps, but projects beyond it on either side. It is overlapped on its inner side by the pronator radii teres, and on the outer side by the supinator longus and extensor carpi radialis longior. Its deep surface is closely connected to the anterior ligament of the elbow

joint. Its chief nerve of supply, from the musculo-cutaneous, has already been secured, but it also receives one or two small twigs from the musculo-spiral, which are given off under cover of the supinator longus.

Dissection.-Separate the supinator longus muscle from the brachialis anticus, and dissect out the musculo-spiral nerve, and the anterior terminal branch of the superior profunda artery, which lie deeply in the interval between them. Here also the anastomosis between the superior profunda and the radial recurrent arteries may be made out, in a wellinjected subject; and the twigs which are given by the musculo-spiral nerve to the brachialis anticus, supinator longus, and extensor carpi radialis longior, looked for.

Triangular Space in front of the Elbow (anticubital fossa). This is a slight hollow in front of the elbow-joint. It corresponds to the popliteal space of the lower limb, and within its area the brachial artery divides into its two terminal branches. In the first instance, let the dissector consider the structures which cover it. These have already been removed, and consist of skin, superficial fascia, and deep fascia. In connection with the latter is the semilunar fascia, whilst within the superficial fascia are the medianbasilic and median-cephalic veins, the anterior division of the internal cutaneous nerve and the cutaneous part of the musculo-cutaneous nerve. These structures constitute the coverings of the space.

The space is triangular. Its base is directed upwards, and is usually regarded as being formed by a line drawn between the two condyles of the humerus. Its inner boundary is the pronator radii teres, and its outer boundary the supinator longus. The meeting of these two muscles below constitutes the apex. The boundaries should now be thoroughly cleaned, and then the contents of the space may be dissected.

Within the space, as we have already stated, there is the termination of the brachial artery, and the radial and ulnar branches into which it divides. To the outer side of the main vessel is placed the tendon of the biceps, and to its inner side the median nerve. A quantity of loose

fat is also present. The ulnar artery leaves the space by passing under cover of the pronator radii teres; the radial artery is continued downwards beyond the apex of the space, overlapped by the supinator longus. The median nerve disappears between the two heads of the pronator radii teres, and the tendon of the biceps inclines backwards

between the two bones of the forearm, to reach its insertion into the radial tuberosity.

When the fatty tissue has been thoroughly removed the floor of the space will be revealed. This is formed by the brachialis anticus and the supinator brevis muscles. In this situation the brachialis anticus is closely applied

to the anterior aspect of the elbow-joint, whilst the supinator brevis is wrapped round the upper part of the radius.

Now divide the bicipital fascia, and separate the bounding muscles widely from each other. Other structures come into view, but they cannot, strictly speaking, be regarded as lying within the space proper. They are—(1) the musculo-spiral nerve, the anterior branch of the superior profunda artery, and the radial recurrent branch of the radial artery, lying deeply in the internal between the supinator longus and the brachialis anticus; (2) the anterior branch of the anastomotica magna, and the anterior ulnar recurrent branch of the ulnar artery, placed under cover of the pronator radii teres.

BACK OF THE ARM.

In this region the following are the structures which require to be studied :

1. The triceps muscle.

2. The superior profunda artery, and the musculo-spiral nerve. 3. The inferior profundą artery, and the ulnar nerve.

4. The posterior branch of the anastomotic artery.

5. The subanconeus muscle.

Dissection. The skin has already been removed from the back or the arm. The deep fascia should now be raised from the surface of the triceps muscle, and its three heads cleaned and isolated from each other. To place the muscle on the stretch, the inferior angle of the scapula should be raised as high as possible, and the forearm flexed at the elbow-joint. The musculo-spiral nerve, together with the superior profundi artery, must at the same time receive the attention of the dissector. They should be followed backwards between the heads of the triceps, and all their branches should be carefully preserved.

Triceps (musculus triceps brachii). This muscle occupies the entire posterior osteo-fascial compartment of the upper arm. It arises by a long or middle head from the

scapula, and by two short heads, outer and inner, from the humerus. The fleshy fibres of these three heads join a common tendon, which is inserted into the top of the olecranon process of the ulna. The superficial part of the muscle is, for the most part, formed by the long scapular head and the outer humeral head of the muscle. The inner humeral head is deeply placed; only a very small portion of it appears superficially in the lower part of the arm on each side of the common tendon of insertion.

The long or scapular head (caput longum) of the triceps arises by a flattened tendon, from the rough triangular impression on the upper part of the axillary border and the lower aspect of the neck of the scapula. This tendon takes origin in the interval between the teres minor and subscapularis muscles. From the lower border of the tendon of the latissimus dorsi muscle an aponeurotic slip proceeds, which joins the long head of the triceps upon its inner aspect. This slip has been already alluded to as a remnant of the dorsi-epitrochlearis muscle of the lower animals.

The two humeral heads take origin from the posterior aspect of the humerus, and if it be borne in mind that no fibres arise from the musculo-spiral groove, and that this groove intervenes between the origins of these heads, their connections will be easily understood. The dissector should provide himself with a humerus, and, having first identified the musculo-spiral groove, proceed to map out the areas of attachment of the humeral heads of the triceps as they are exhibited in the dissected part.

The outer head (caput laterale) of the triceps arises from the outer and posterior aspect of the shaft of the humerus, above the level of the musculo-spiral groove. It takes origin, by short tendinous fibres, along a line which descends vertically from the insertion of the teres minor above to the upper border of the musculo-spiral groove below. But it also derives fibres from a strong aponeurotic bridge or arch, which is thrown over the groove, so as to give protection to the superior profunda