This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The bellies of many of the muscles, mainly the superficial ones, cease as they become tendinous about the middle of the forearm. Hence the rapid decrease in size as one descends. When the wrist is reached there is a swelling on each side caused by the expanded lower end of the radius on the outer side and the head of the ulna on the inner. The medial (inner) prominence is rendered more marked by abducting the hand, the lateral (outer) prominence by adducting it. Just beyond these there is a constriction as the wrist passes into the hand.
Above the wrist on the anterior and outer part can be felt the radius. Its lower 2 or 2.5 cm. (1 in.) is sharp and prominent - this is the anterior border of the styloid process. On the outer side at its base is the point of insertion of the brachioradialis tendon. Following the bone down on its outer side, at the upper margin of the anatomical snuff-box, one feels the tip of the styloid process, a most important landmark.
On the outer surface of the radius beginning below between the tip of the styloid process and its sharp anterior border are the extensor ossis metacarpi pollicis and extensor brevis pollicis tendons. They can readily be seen and felt when the thumb is extended as they cross obliquely over the lower end of the radius. The sheaths of these tendons frequently become inflamed from injuries, causing what is termed tenosynovitis. If the hand is laid on the lower portion of the radius of a patient so affected, and he is told to move the thumb, a characteristic creaking can be felt as the tendons move in their inflamed sheaths.
The edge of the articular surface of the radius can be indistinctly felt from the tip of the styloid process to the edge of the flexor carpi radialis internally and across the back of the wrist in an upwardly curved line toward the ulna.
On the inner side of the wrist can be felt and seen the prominence made by the head of the ulna. The ulna is subcutaneous and can be followed up the forearm posteriorly its entire length. It is not covered by muscles on its inner border, but on its anterior surface is the flexor carpi ulnaris tendon beneath which is the flexor profundus digitorum, this latter being separated from the bone by the origin of the pronator quadratus. If the posterior surface of the ulna is followed downward the styloid process forming its extremity can be distinctly felt, especially if the hand is placed in the supine position and slightly flexed. Overlying the head of the ulna posteriorly is the tendon of the extensor carpi ulnaris muscle going to the base of the fifth metacarpal bone. This tendon follows the movements of the hand in pronation and supination, but the styloid process of the ulna remains stationary. When the hand is pronated the tendon lies to the anterior side of the styloid process, but when the hand is supinated it lies toward its posterior side. This tendon cannot be readily recognized.
The inner and posterior surface of the cuneiform bone can be felt immediately below the head of the ulna. Some difficulty may be experienced in distinguishing one from the other; if, however, the hand is abducted and adducted the cuneiform bone can be felt to move while the ulna remains stationary. On the palmar surface of the wrist, immediately below the ulna, can be felt the distinct bony prominence formed by the pisiform bone. The flexor carpi ulnaris inserts into it.
About 2 to 2.5 cm. (1 in.) below and to the radial side of the pisiform bone is the unciform process of the unciform bone. It is best detected by laying the ball of the thumb over the spot and making deep pressure with a rolling motion. On the radial side of the anterior surface, directly in line with the tendon of the flexor carpi radialis, is the prominent tubercle of the navicular (scaphoid) bone; a centimetre farther on, in line with the thumb, is the ridge of the trapezium. The anterior annular ligament is attached to its outer surface about 2.5 cm. (1 in. ) below the styloid process of the radius; a bony prominence formed by the trapezium marks its junction with the metacarpal bone of the thumb in front.
The ability to locate the carpometacarpal joint of the thumb is of importance in reference to the diagnosis of fractures and other injuries. On comparing the two styloid processes it will be seen that the styloid process of the radius extends 1 cm. (2/5 in.) lower than that of the ulna. This is best observed with the hand in a prone position. Across the front of the wrist there are two transverse lines. The proximal or upper one corresponds with the radiocarpal joint or wrist-joint. The distal or lower one corresponds with the joint between the two rows of carpal bones and marks the upper edge of the anterior annular ligament.
On the posterior surface of the wrist, one-third of the width of the wrist across from the edge of the radius, can be felt a bony prominence. It is the posterior radial tubercle. If the thumb is extended the tendon of the extensor longus pollicis leads directly to the tubercle and lies along its ulnar border. This tubercle marks the middle of the posterior surface of the radius. The radius passes two-thirds across the wrist and the ulna the other third; by firm pressure the interval between them can be felt.
If the hand is firmly clenched and flexed on the forearm the tendons on the anterior surface of the wrist become prominent. The most evident is the palmaris longus which, though sometimes absent, usually stands out clear and shap. Lving along its radial border is the tendon of the flexor carpi radialis; between the two on a lower level lies the median nerve. In front of the ulna, and going directly downward to the pisiform bone, is the tendon of flexor carpi ulnaris (Fig. 349).
Fig. 349. - Surface anatomy of anterior surface of wrist.
Fig. 350. - Surface anatomy of the anterior portion of the wrist.
If the hand is extended the tendon of the flexor carpi ulnaris stands out clearly. In the hollow to its lateral (outer) side lie the ulnar nerve and artery. A rounded muscular swell fills the space between the ulnar artery and the tendon of the palmaris longus, - it is caused by the flexor sublimis digitorum (see Fig. 350). It is here that abscesses show when they travel up from the hand.
Between the outer edge of the flexor carpi radialis tendon and the anterior outer edge of the radius is a groove in which runs the radial artery. The position of the extensor ossis metacarpi pollicis and extensor brevis pollicis which run together over the outer surface of the radius can best be determined by abducting the thumb and so making these tendons prominent (Fig 351).
Fig. 351. - Surface anatomy of the outer dorsal portion of the wrist, showing the anatomical snuff-box.
In the same manner the extensor longus pollicis tendon can be made prominent and followed to the posterior radial tubercle. By firm pressure the upper limits of the first and second interosseous spaces can be felt. They mark the bases of the metacarpal bones. The extensor carpi radialis longior passes across the snuff-box to insert into the radial side of the base of the second metacarpal bone. The radial artery as it dips down between the first and second metacarpal bones lies just to its outer side. Crossing under the tendon of the extensor longus pollicis is the extensor carpi radialis brevior, which proceeds to the top of the second interosseous space to insert into the adjoining sides of the second and third metacarpal bones.
In the chink between the radius and ulna lies the tendon of the extensor minimi digiti. Between this tendon and the radial tubercle are the four tendons of the extensor communis digitorum and extensor indicis muscles. Passing over the head of the ulna to insert into the base of the fifth metacarpal bone is the tendon of the extensor carpi ulnaris. It is best felt just beyond the extremity of the ulna when the hand is drawn toward the ulnar side. It inserts into the base of the fifth metacarpal bone.