artery and the musculo-spiral nerve. The strength and position of this arch can be tested by thrusting the handle of the knife downwards and outwards in the musculospiral groove, and along the course of the nerve and artery under the external head of the triceps. By its lower end the arch is connected with the external intermuscular septum.

The inner head (caput mediale) of the triceps is placed below the musculo-spiral groove. It sends upwards, on the posterior aspect of the humerus, and along the inner margin of the groove, a narrow pointed fleshy slip, which obtains origin from the bone as high as the insertion of the teres major muscle. Below, it widens out and arises by short fibres from the entire breadth of the posterior surface of the humerus. It also springs from the posterior surface of the internal intermuscular septum, and from the lower part of the corresponding surface of the external intermuscular septum. The inner head of the triceps, therefore, has very much the same origin from the back of the bone that the brachialis anticus has from the front of the bone.

The dissector should now study the common tendon of insertion of the triceps. The long and the outer heads end in a broad, flat tendon, which is inserted into the back part of the upper surface of the olecranon process, and at the same time gives off, on the outer side, a strong expansion to the fascia of the forearm as it covers the anconeus muscle. The short fleshy fibres of the inner head are, for the most part, inserted into the deep surface of the common tendon, but a considerable number find direct attachment to the olecranon, whilst a few of the deepest fibres are inserted into the loose posterior part of the capsule of the elbow-joint. These latter fibres have been described as a separate muscle under the name of subanconeus.

Dissection. In order that the musculo-spiral nerve and the superior profunda artery, may be fully exposed, the external head of the triceps must be divided. Thrust the handle of a knife along the musculoVOL. I.-6

spiral groove, and under the muscle. This will give the direction in which the outer head of the triceps should be severed. Beyond cleaning the nerve and its branches, and the superior profunda artery, as they lie in the groove, no further dissection is necessary.

The Musculo-spiral Nerve (nervus radialis) is the direct continuation of the posterior cord of the brachial plexus after it has furnished in the axilla the three subscapular and the circumflex nerves. In the first instance, the musculospiral proceeds downwards behind the lower part of the axillary artery and the upper part of the brachial artery. It soon leaves the front of the arm, however, and, inclining backwards with the superior profunda artery, enters the interval between the long and the inner heads of the triceps, and reaches the musculo-spiral groove. In this it is conducted round the back of the shaft of the humerus, under cover of the outer head of the triceps, and on the outer side of the limb it pierces the external intermuscular septum and appears in the anterior compartment of the arm. Here it has already been exposed. It lies deeply in the interval between the brachialis anticus on the inside, and the supinator longus and extensor carpi radialis longior on the outside. It ends in front of the external condyle of the humerus by dividing into two terminal branches, viz., the radial and the posterior interosseous. The musculo-spiral nerve presents therefore very different relations as it is traced from its origin to its termination: (1) between the subscapularis, latissimus dorsi, teres major, and long head of the triceps which support it behind, and the axillary and brachial arteries which are placed in front of it; (2) between the long and inner heads of the triceps; (3) in the musculo - spiral groove between the bone and the outer head of the triceps; (4) in the interval between the brachialis anticus on the inside, and the supinator longus and extensor carpi radialis longior on the outside.

The branches which proceed from the musculo-spiral nerve are partly muscular and partly cutaneous.

The cutaneous branches are three in number, and have already been traced. They are-(1) the internal cutaneous,

which, as a rule, arises within the axilla, in common with the branch which supplies the long or scapular head of the triceps; (2) the upper external cutaneous, and (3) the lower external cutaneous, which come off on the outer side of the arm close to the outer margin of the external intermuscular septum (p. 61).

The muscular branches go to the three heads of the triceps, to the anconeus, to the brachialis anticus, to the supinator longus, and to the extensor carpi radialis longior. The branches to the three last muscles spring from the main trunk after it has pierced the external intermuscular septum.

The branch to the inner head of the triceps is sometimes termed the ulnar collateral nerve. It is a long slender filament, which runs downwards to supply the lower fibres of the inner head of the triceps, and it receives this name from the close manner in which it is applied to the ulnar nerve in the lower part of its

course.

The branch to the anconeus is also a long slender twig, which enters the substance of the internal head of the triceps, and appears at first sight to terminate there, but, if traced downwards, it will be found to end in the

anconeus.

Superior Profunda Artery (arteria profunda brachii).— This artery has been already observed to take origin from the brachial trunk, immediately below the lower margin of the teres major muscle. It accompanies the musculo-spiral nerve, and its relations to the three heads of the triceps and the musculo-spiral groove of the humerus are exactly the same as those of the nerve. When it reaches the external intermuscular septum, at the outer side of the arm, it ends by dividing into two terminal branches—an anterior and a posterior. anterior and smaller branch accompanies the musculo-spiral nerve through the septum, and follows it downwards to the anterior aspect of the external condyle of the humerus, where it anastomoses with the radial recurrent

The

artery. The posterior larger branch proceeds downwards on the posterior surface of the external intermuscular septum, and anastomoses on the back of the external condyle of the humerus with the posterior interosseous recurrent artery.

The branches which proceed from the superior profunda artery are chiefly distributed to the three heads of the triceps muscle. One twig runs upwards between the long and outer heads of the muscle, and anastomoses with the posterior circumflex artery. In this way, a link is established between the axillary and brachial systems of branches.

Dissection. The ulnar nerve, with the inferior profunda artery, and the slender ulnar collateral nerve, can now be advantageously followed, as they proceed downwards upon the posterior aspect of the internal intermuscular septum. They are covered by a thin layer of fleshy fibres belonging to the internal head of the triceps. The posterior branch of the anastomotica magna, after it has pierced the internal septum, should also be dissected out. As a rule, a transverse branch passes between this vessel and the posterior terminal part of the superior profunda. It lies upon the back of the humerus, immediately above the elbow-joint, and can be exposed by dividing the triceps muscle a short distance above the olecranon. At the same time the fleshy fibres of the internal head of the triceps, which are inserted into the posterior ligament of the joint, and constitute the subanconeus muscle, should be examined. Lastly, raise the lower piece of the triceps from the elbow-joint, and look for a small bursa between the deep surface of the triceps tendon and the upper aspect

of the olecranon.

SHOULDER-JOINT (ARTICULATIO HUMERI).

Before proceeding to the dissection of the forearm it is advisable to study the shoulder-joint, because if this is deferred too long the ligaments are apt to become dry.

In no joint in the body is the movement so free, and so varied in its character, as in the shoulder-joint. This is rendered necessary by the many functions which are performed by the upper limb.

Freedom of motion is provided for in two ways-(1) by the large size of the head of the humerus, in comparison with the small dimensions and shallow character of the glenoid fossa-the socket in which it moves; (2) by the great laxity of the ligamentous structures which connect the humerus with the scapula.

These

FIG. 24.-Coronal or vertical transverse section through the
left shoulder-joint. (Viewed from behind.)

provisions for allowing an extensive range of movement at this articulation might, at first sight, lead one to doubt the security of the joint. Its strength certainly does not lie in the adaptation of the bony surfaces to one another, nor in the power of its ligaments.