common extensor and the tendon of the extensor indicis.

The fifth compartment is situated over the interval between the lower ends of the radius and ulna. It contains the slender tendon of the extensor minimi digiti. The sixth and innermost compartment, which corresponds with the groove on the back of the lower end of the ulna, encloses the tendon of the extensor carpi ulnaris.

FIG. 32.-Diagrammatic section through the lower end of the forearm to show the compartments of the posterior annular ligament. (From Reeves, slightly modified.)

1. First compartment, with the tendons of the extensor ossis and extensor primi internodii pollicis.

2. Second compartment, with the tendons of the two radial extensors.

3. Third compartment, with tendon of extensor secundi internodii pollicis.

4. Fourth compartment, with the tendons of the extensor communis and
extensor indicis.

5. Fifth compartment, with the tendon of the extensor minimi digiti.
6. Sixth compartment, with the tendon of the extensor carpi ulnaris.

Extensor Tendons of the Fingers.-The four tendons of the extensor communis digitorum, when they emerge from their compartment in the posterior annular ligament, diverge on the dorsum of the hand to reach the four fingers. The tendon of the ring finger will be seen to be connected by a tendinous slip with the tendon on either side of it. This explains the small degree of independent movement in a backward direction which the ring digit possesses. The arrangement of the tendons on the fingers is the same in each case. Upon the dorsal aspect of the first phalanx the tendon expands so as to cover it completely. Into the margins of this "dorsal expansion," the delicate

tendons of the lumbrical and interosseous muscles are inserted. Near the first interphalangeal joint the expansion separates into three portions-a central and two lateral. The central part, which is the weakest, is inserted into the dorsal aspect of the base of the second phalanx. The stronger lateral portions unite into one piece beyond this, and gain an insertion with the base of the ungual phalanx.

The tendon of the extensor indicis joins the expansion of the extensor tendon on the dorsal aspect of the first phalanx of the index finger.

The tendon of the extensor minimi digiti splits into two parts. Of these the outer joins the tendon of the common extensor which goes to that digit, whilst the inner ends in the dorsal expansion.

Posterior Interosseous Nerve. The terminal filament

of this nerve can now be traced downwards to the dorsal aspect of the carpus. It passes under cover of the extensor indicis, the tendons of the extensor communis, and the posterior annular ligament. On the carpus it ends in a gangliform swelling, from which proceeds fine twigs for the supply of the numerous joints in the vicinity.

Dissection. The limb should now be turned round, so that the

transverse metacarpal ligament which stretches across the palmar surface of the heads of the metacarpal bones may be examined previous to the dissection of the interosseous muscles.

Transverse Metacarpal Ligament.-The transverse metacarpal ligament is a strong band composed of transverse fibres, which is placed upon the palmar aspect of the heads of the four metacarpal bones of the fingers. Commencing on the outer side upon the distal extremity of the index metacarpal, it ends at the inner margin of the hand upon. the head of the metacarpal bone of the little finger. It is not directly attached to the bones, but is fixed to the powerful anterior ligaments of the four inner metacarpophalangeal joints, and it effectually prevents excessive separation of the metacarpal bones from each other.

Dissection. To obtain a satisfactory view of the interosseous muscles the adductor transversus pollicis, if not previously reflected, should be detached from its origin, and thrown outwards towards its insertion into the thumb. The transverse metacarpal ligament must also be divided in the intervals between the fingers.

The Interosseous Muscles occupy the intervals between the metacarpal bones. They are seven in number; and are arranged in two groups, viz., a dorsal and a palmar.

The dorsal interossei (musculi interossei dorsales) are four in number, and are more powerful than the palmar muscles. They are best seen on the dorsal aspect of the hand, but they are also visible in the palm. They act as abductors of the fingers from a line drawn through the middle digit, and their insertions are arranged in accordance with this action. Each muscle arises by two heads from the contiguous surfaces of the two metacarpal bones between which it lies, and the fibres converge in a pennate manner upon a delicate tendon. In the case of the first or outermost dorsal interosseous muscle, this tendon is inserted into the radial side of the base of the first phalanx, and also into the radial margin of the dorsal expansion of the extensor tendon of the index. The second and third dorsal interosseous muscles are inserted in a similar manner upon either side of the base of the first

phalanx of the middle finger; whilst the fourth has a corresponding insertion upon the ulnar aspect of the base of the first phalanx of the ring finger.

The first dorsal interosseous muscle is frequently termed the abductor indicis, and between its two heads of origin the radial artery enters the palm. Between the heads of the other three muscles the small posterior perforating arteries pass.

The three palmar interosseous muscles (musculi interossei volares) can only be seen on the palmar aspect of the hand. They act as abductors of the index, ring, and little fingers towards the middle digit, and each muscle is placed upon the metacarpal bone of the finger upon which it acts. The first palmar interosseous muscle therefore arises from the metacarpal bone of the index finger, and its delicate tendon is inserted upon the ulnar side of that digit, partly into the base of the first phalanx, and partly into the extensor expansion. The second palmar interosseous muscle springs from the metacarpal bone of the ring finger, and has a similar insertion into the radial side of that digit. The third palmar interosseous muscle takes origin from the metacarpal bone, and presents a corresponding insertion into the radial side of the first phalanx and extensor expansion of the little finger. The interosseous muscles are supplied by the deep branch of the ulnar nerve.

Deep Head of the Flexor Brevis Pollicis (interosseous primus volaris of Henle).—This minute muscle can best be displayed from the dorsal aspect of the hand by reflecting the radial head of the first dorsal interosseous muscle. It arises from the base of the metacarpal bone of the thumb, and is inserted into the ulnar sesamoid bone of that digit. It is deeply placed, and is entirely covered from the front by the adductor obliquus pollicis.

Tendon of the Flexor Carpi Radialis.-The tendon of this muscle should now be traced through the groove on the front of the trapezium to its insertion into the base of the metacarpal bone of the index. It presents also a minor attachment to the base of the middle metacarpal bone.

Dissection.-All the muscles around the elbow-joint should be removed. In raising the brachialis anticus and the triceps from the front and back of the articulation, some care is required to avoid injury to the anterior and posterior ligaments. It is advisable to remove the supinator brevis last, because it is only when this muscle is completely isolated that a proper idea of its attachments and mode of action can be obtained.

Supinator Radii Brevis.-The supinator brevis envelops the upper part of the shaft and the neck of the radius, covering it completely, except on its inner side. It arises from the deep depression below the lesser sigmoid cavity of the ulna, and also from the external lateral ligament of the elbow and the orbicular ligament of the radius. From this origin the fibres sweep round the posterior, outer and anterior surfaces of the radius, and clothe its shaft as far down as the insertion of the pronator radii teres. The posterior interosseous nerve traverses the substance of the muscle, and separates it into two layers.

ARTICULATIONS.

Elbow-joint (articulatio cubiti).-At the elbow-joint the. trochlear surface of the humerus is grasped by the greater

elbow-joint.

sigmoid cavity of the ulna. The shallow depression on the upper surface of the head of the radius rests upon the

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