Although the lateral width of this muscle is considerable, only a very narrow edge is in contact with the fascia, the remainder being covered by the last-mentioned muscles, so that some anatomists have described it as constitu ting a middle layer.
On the internal edge is placed the flexor carpi ulnaris, which maintains the same relative position to it throughout the fore-arm. In contact with its posterior face we have the flexor digitorum profundus, the flexor longus pollicis, and the ulnar artery, vein, and nerve.
This muscle flexes the second phalanx on the first, and the first on the metacarpus, and the wbole hand on the fore-arm.
5. Flexor carpi ulnaris, (musculus ulnaris internus, Soemm, cubital interne, Portal, cc bito-carpien, Chauss.) This muscle arises from the internal extremity of the internal condyle of the humerus from the tendinous intermuscu lar septum, between it and the flexor carpi digi torum sublimis, and from the olecranon process of the ulna; between these two heads the ulnar nerve is situated; its origin from the ulna is not limited to the olecranon process, for it con tinues its connexion with that bone nearly as low down as the origin of the pronator quad ratus. This muscle, which arises tendinous and fleshy, merges into tendinous fibres on its anterior surface at the upper part of the lower third of the fore-arm. The tendon by dc2,rees becomes more rounded, but does not cease to receive fleshy fibres until it terminates by be coming inserted into the annular ligament and ospisiforme.
The flexor carpi ulnaris, forming the inner margin of the muscles of the fore-arm, is in contact with the fascia: its external edge touches the flexor communis digitorum sublimis. The relation of this muscle to the ulnar artery has induced some anatomists to denominate it muscle satellite de l'art?re cubitole.
In addition to its power as a flexor of the hand on the fore-arm, this muscle adducts the hand, drawing it towards the mesial line.
b. The deep layer (f muscles.—These are three in number, the flexor longus proprius pollicis, flexor communis digitorum prqlthohes perforans, and the pronator quadrates. A por
tion of the supinator radii brevis is also found in it.
1. The flexor longus proprius pollicis, Scernm. (Radio-phalangettien du ponce, Chauss.) This muscle, situated most exter nally, arises by two heads ; one, narrow, rounded, tendinous, and fleshy, from the inner condyle of the humerus; the other, broad and fleshy, from the front of the radius, below the insertion of the biceps and supinator radii brevis, and from the interosseal ligament, extending as low down as the insertion of the pronator quadratus. Its tendon, first formed on its internal and anterior edge, descends behind the annular ligament of the wrist-joint, and taking its course between the two heads of the flexor brevis pollicis, is inserted into the last phalanx of the thumb.
This muscle is covered anteriorly by the supinator radii longus and extensor carpi radialis longior, except at the lower part, where it is simply covered by the deep fascia on which the radial artery lies. To its inner side is the flexor digitorum profundus.
This muscle is a flexor of the last phalanx of the thumb, a powerful and important muscle in grasping objects.
2. Flexor communis digitorum profundus poforans. (M. poforans, Swam. 1l1. cubito phalangettien common, Chauss.) arises tendi nous from the front of the ulna immediately below the insertion of the brachialis anticus into the tubercle of that bone, and from the same as low down as the pronator quadratus ; also from the interosseous ligament. It becomes tendinous on its anterior face about the middle of the fore-arm, thus presenting a smooth and polished surface to the muscles in front of it : like the superficial flexor, it forms its four tendons, which, after traversing the palm of the hand and piercing the split tendons of the superficial flexor, are ultimately inserted into the third phalanx of each of the fingers.
This muscle has the flexor longus pullicis to its outer side ; the flexor carpi ulnaris to its inner ; and the flexor carpi radialis, flexor com munis digitorum sublimis and palmaris longus, anterior to it.