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 deep fascia of the forearm is continuous with that of the arm. It forms a complete covering for the muscles and sends septa between them. It is especially strong posteriorly. It is attached to the medial and lateral condyles of the humerus, the sides of the olecranon process and the whole length of the ulna posteriorly. Below the medial condyle anteriorly it is strengthened by the bicipital fascia. In the antecubital fossa it is pierced by a large communicating vein which connects the superficial and deep veins. Toward its lower end posteriorly, it is strengthened by transverse fibres and becomes attached to the longitudinal ridges on the radius and blends with the posterior annular ligament.
Below anteriorly it is thin and forms a covering for the tendons of the palmaris longus and flexor carpi radialis muscles and at the wrist blends with the annular ligament beneath. This latter, as pointed out by Davies Colley ("Morris's Anatomy," page 311), is a continuation of the layer of fascia covering the flexor sublimis digitorum.
When infection involves the deep tissues of the forearm the pus, being hindered from going externally by the fibrous septa between the various layers of muscles as well as the deep fascia itself, tends to burrow up and down the arm. If in the upper portion of the forearm, it tends to point in the antecubital fossa. If lower down, it tends to come to the surface on the radial side between the flexor carpi radialis and brachioradialis or toward the ulnar side between the palmaris longus and flexor carpi ulnaris.
the tendons of the palmaris longus and flexor carpi radialis and the median nerve, - form a solid barrier anteriorly which inclines the pus to one side. Above posteriorly it may work its way upward behind the internal condyle, following the ulnar nerve.
The fibrous septa of the various muscles hinder the progress of pus laterally, and the attachment of the deep fascia to the ulna prevents its passing around the arm at that point. The many pockets formed by the pus in its burrowing between the muscles render these abscesses difficult to drain and tedious in healing.
Should infection from the thumb travel up the flexor longus pollicis tendon, when it reaches above the wrist it is directly beneath the tendon of the flexor carpi radialis. In such a case an incision should be made along the radial (outer) edge of the tendon, taking care not to wound the radial artery still farther out. If pus infects the forearm by following up the flexor tendons of the fingers beneath the anterior annular ligament, it shows itself above the wrist between the palmaris longus and flexor carpi ulnaris tendons and can here be incised. If it is desired to introduce a drain beneath the flexor muscles, an incision may be made along the side of the ulna and a forceps passed under the flexor tendons and made to project under the skin of the radial side where a counter opening can be made and the drain inserted. (For a discussion of the treatment of purulent affections of the hand and forearm see A. B. Kavanel: " Surgery, 'Gynecology, and Obstetrics," 1909, p. 125, vol. viii, No. 3.) Suppuration around these tendons is very serious, as the effusion binds together the tendons and irritates the nerves and produces disabling contractures which are exceedingly difficult to remedy.