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.
As the tendons pass down over the posterior surface of the radius and ulna they are bound down by processes of the deep fascia which form canals in which they run. The deep fascia of the posterior surface of the forearm in the neighborhood of the wrist is strong, and forms the posterior annular ligament. Its lower border is about level with the upper border of the anterior annular ligament. It is attached externally to the posterior and outer edge of the styloid process of the radius and internally to the posterior surface of the styloid process of the ulna, the internal lateral ligament, the pisiform, and adjacent carpal bones. Beneath this posterior annular ligament are six compartments. From the radial toward the ulnar side they are: (1) One on the outer side of the styloid process of the radius for the extensor ossis metacarpi pollicis and extensor brevis pollicis; (2) for the extensor carpi radialis longior and brevior, then comes the posterior radial tubercle in the middle of the radius, and passing close along its ulnar side is (3) the extensor longus pollicis. To the ulnar side of this tendon is a comparatively broad sheath for (4) the extensor communis digitorum and the extensor indicis muscles. In the interval between the radius and ulna lies (5) the tendon of the extensor minimi digiti, and on the posterior side of the styloid process of the ulna is (6) the tendon of the extensor carpi ulnaris (Fig. 347).
Fig. 346. - Muscles of the radial side of the wrist.
Each of these six compartments is lined with a separate sheath which extends under the annular ligament from a centimetre or two above the joint to about the bases of the metacarpal bones on the dorsal surface of the hand.
On the outer dorsal aspect of the wrist, just below the radius, is a depression particularly noticeable when the thumb is abducted (Fig. 351, page 341). It is triangular in 22 shape with its base upward. The styloid process of the radius forms its base; the extensor brevis pollicis with the extensor ossis metacarpi pollicis forms its radial or outer side, and the tendon of the extensor longus pollicis forms its ulnar or inner side. Its floor is formed by the navicular (scaphoid) and trapezium bones. Through it, lying on these bones and the external lateral ligament, passes the radial artery on its way to the first interosseous space. Superficial to the artery lies a vein and some fine branches of the radial nerve. In ligating the artery at this point, care should be taken not to mistake the vein for it; the vein is near the skin, the artery lies deep on the lateral ligament and bones (Fig. 348).
Fig. 348. - View of the anatomical snuff-box and the radial artery passing through it.