The hand is twice as long as it is broad. The length of the middle finger from the metacarpophalangeal joint to its extremity is equal to the distance from the metacarpophalangeal joint to the radiocarpal joint. If the hand is turned with the palm up, the thumb diverges from the median line at an angle of 40 degrees. The palm is hollow, with a muscular mass on each side. That on the thumb side is called the thenar eminence; it is formed by the abductor, opponens, and outer head of the flexor brevis pollicis. The prominence on the ulnar side of the hand is called the hypothenar eminence and is formed by the abductor, opponens, and flexor brevis minimi digiti. The palmaris brevis muscle overlies them transversely. The palm is marked by four creases, two longitudinal and two transverse. One longitudinal crease begins at the middle of the wrist between the thenar and hypothenar eminences to end on the radial side of the index finger, opposite the head of its metacarpal bone. It is caused by adduction of the thumb. The other longitudinal crease runs somewhat parallel to the first, starting near the wrist and ending in the web between the index and middle fingers. It is formed by hollowing the hand. The upper transverse crease begins on the radial side of the index finger where the first longitudinal crease ends, and runs obliquely across the palm to the middle of the hypothenar eminence. It is formed by the flexion of the fingers, especially the index, and where it crosses a line drawn through the middle of the middle finger marks the lowest point of the superficial palmar arch. The position of the lowest portion of the superficial palmar arch is also indicated by a line drawn across the palm opposite to the web of the thumb and index finger. The lower transverse crease begins on the hypothenar eminence opposite the head of the fifth metacarpal bone and is formed by the flexion of the middle, ring, and especially the little finger. When it reaches the vicinity of the median line it merges with the second longitudinal crease which passes to the web between the index and middle fingers. Midway between the crease and the webs of the fingers lie the joints of the middle, ring, and little finger. More stress is apt to be laid on a knowledge of these creases than they deserve (Fig. 369).

Fig. 368.   Muscles of the hand.

Fig. 368. - Muscles of the hand.

The position of the metacarpophalangeal joints is best determined by feeling for them on the dorsum of the hand and then taking a corresponding point on the palmar surface. They are sufficiently accurately located by taking a point 2 cm. (3/4 in) behind the web of the fingers. The creases for the middle phalangeal joints are directly opposite the articulations. The creases for the end phalangeal joints are to the proximal side of the articulations. The deep palmar arch lies about 1.5 cm. (3/5 in.) closer to the wrist than the superficial.

The digital arteries from the superficial palmar arch pass downward with the digital nerves, superficially, in the spaces between the metacarpal bones, to the webs of the fingers. About 1 cm. (2/5 in.) behind the web they sometimes receive branches from the deep palmar arch, and then divide to go to each lateral palmar side of the fingers. The palmar fascia divides into its four slips just below the line of the superficial palmar arch, opposite the web of the thumb.

Fig. 369.   The palmar surface of the hand showing thenar and hypothenar eminences and creases.

Fig. 369. - The palmar surface of the hand showing thenar and hypothenar eminences and creases.

On the dorsum of the hand the extensor tendons can be seen. Accessory slips usually connect the tendon of the ring finger with that of the little finger and middle finger. A slip also usually passes from the tendon of the middle to that of the index finger.

The slip from the tendon of the ring to that of the little finger has been thought to restrict the freedom of the movement of the ring finger, hence in musicians it sometimes has been divided. The operation is done by first flexing the fingers, which brings the slip well forward near the knuckle, and then introducing a thin knife longitudinally beneath it and cutting toward the skin. The procedure has not found favor among musicians. '

The metacarpal bones are subcutaneous and can readily be felt their entire length. The muscular prominence on the dorsum of the hand seen when the thumb and forefinger are approximated is due to the abductor indicis muscle. At its upper, extremity the radial artery passes between its two heads to enter the palm. When the thumb is extended the snuff-box becomes evident and the extensor longus pollicis tendon is distinctly seen leading to the ulnar side of the posterior radial (thecal) tubercle on the middle of the dorsum of the radius. The tendons on the radial side of the snuff-box are the extensor brevis and extensor ossis metacarpi pollicis.

When the fingers are flexed, the prominence of the knuckles is formed by the proximal bones; the distal phalanges fold under the proximal ones and the joint line is about 1 cm. (2/5 in.) below the dorsal surface of the metacarpal bones (Fig. 366, page 353).

Fig. 371.   The arteries of the hand.

Fig. 371. - The arteries of the hand.

inner or ulnar side. As it enters the hand it lies just to the radial side of the pisiform bone with the nerve intervening. Both the artery and nerve lie on the anterior annular ligament. As soon as they pass the pisiform bone they go under the small palmaris brevis muscle and the palmar fascia, and lie on the flexor tendons.

The artery then describes a curve across the palm of the hand toward the web of the thumb. It crosses the middle of the third metacarpal bone at or a little above the level of the web of the thumb and continues on to the radial side of the metacarpal bone of the index finger. Here it receives the superficial volar artery from the radial as well as a communicating branch from the princeps pollicis and radialis indicis. When one of these branches is large the other two are smaller or lacking altogether. Not infrequently the communication with the radial at this point is in the form of a large branch which passes superficially across the web of the thumb and index finger, and its pulsations can be both seen and felt (Fig. 372).