FORE-ARM, MUSCLES OF THE.— When we consider how varied and complex are the motions of the arm and hand, it is no matter of surprize that so many as nineteen muscles should be found composing the fleshy mass of the fore-arm.
These muscles may be classified in reference to their action, and are briefly enumerated as follows:— I n the first place there is one muscle physio logically belonging to the upper arm, the anco liens ; the rest are connected with the hand; for instance, there are three flexors of the hand ; Ileror carpi radialis, flexor carpi ulaaris, pal maris fungus. Three extensors of the hand, extensor carpi radialis longior, extensor carpi radialis brevior, extensor carpi ultiaris. Three long flexors of the thumb and fingers; flexor communis digitorum sublimis, flexor communis digitorum prifundus, flexor longus proprius pollicis. Five extensors of the thumb and fingers, extensor ossis metacarpi ;minds, exten sor priini internodii, extensor secundi internodii, extensor communis digitorum, extensor indicis.
Two supinators, supinator radii supina tor radii brcvis. Two pronators, pronator ra dii tcrcs, pronator quadratas.
In proceeding to describe the attachments and relations of the foregoing muscles, it will be found convenient to examine them as they are met with in the following regions of the fore-arm. 1. The anterior region, which con tains a superficial and a deep set of muscles. 2. The posterior region, which likewise has its superficial and its deep layers of muscles. These regions again may be conveniently sub divided into radial and ulnar sections, between which a very natural line of demarcation is observable after the skin and adipose tissue have been removed.
Exactly on this line, and one-third from the elbow and two-thirds from the wrist-joint, two long muscles will be found in contact, the supi nator radii longus in the radial section, the flexor carpi radialis in the ulnar. Above and below this point these muscles diverge. The flexor carpi radialis, at its origin from the inter nal condyle, is distant, from the boundary line referred to, at least one-third of the transverse diameter of the arm, and the space thus left, triangular in its figure, contains a large portion of the pronator radii teres. The radial edge of
the pronator radii teres is in contact with the supinator radii longus to the extent of about an inch and a half; this muscle descending in like manner obliquely from its origin in the ulnar section towards the radial leaves above a similar though small triangular space, in which the tendon of the biceps flexor cubiti is lodged. Below the point referred to above, between the elbow and wrist, the flexor carpi radialis runs in contact with and on the ulnar side of the boundary line till within an inch and a half of the wrist-joint, where it gradually slides into the radial section, so that at the annular liga ment the tendon of the flexor carpi radialis will be found entirely in the radial region with its internal edge in contact with the boundary line.
Thus it will be seen that a line drawn from the elbow to the wrist and dividing the fore arm into two portions, of which the internal or ulnar section is exactly two-thirds, while the external or radial section is only one-third of the transverse width, not merely forms an arti ficial division into radial and ulnar sections, but also points out the exact situation of the tendon of the biceps, the outer edge of the pronator radii teres, and the flexor carpi radi alis, and in addition, as we shall presently see, the supinator radii longus. The fleshy belly of this muscle lies exactly parallel with this boundary line in the upper half of the arm.
In consequence of the supinator radii longus becoming tendinous about the middle of the fore-arm, and the tendon being narrower in its transverse diameter than the muscle, a space is left at the lower part of the arm between it and the flexor carpi radialis, and the supinator radii longus is no longer met in contact with the boundary line. In this space is lodged the radial artery, lying midway between these two tendons, separated from the flexor longus pol licis by a deep layer of fascia, which is united to the edge of the supinator radii longus on the outer side, and the flexor carpi ulnaris on the inner side