Now we’ll move on to look at the intrinsic muscles of the hand. They’re in four groups: the interosseous muscles, the lumbricals, the short muscles of the thumb, and the short muscles of the little finger.
There are seven interosseous muscles, or interossei. Here they all are together. There’s one for each side of the index, middle, and ring fingers, and one for the radial side of the little finger. By tradition they’re divided into these four dorsal interossei, and these three palmar ones, but to understand what they do, it’s simpler to consider them in twos, like this.
The two interossei for the middle finger are a typical pair. They arise from the shaft of their own metacarpal, and from its neighbors. They pass behind the deep transverse metacarpal ligament. We’ll remove the ligament, and we’ll also remove the other fingers and metacarpals to simplify the picture. On each side of the MP joint, the interosseous muscle narrows down to a double tendon, which has a long part, and a short part.
The short part inserts here on the base of the proximal phalanx. The long part of the interosseous tendon joins the extensor mechanism to become its most outlying part. Merging with the lateral slip of the extensor tendon, it forms the lateral band of the extensor mechanism. The two lateral bands come together distally, as we’ve seen, to insert here on the distal phalanx.
The line of action of the interosseous tendon passes in front of the axis of rotation of the MP joint, marked by this pin, and behind the axes of the two IP joints. When the two interosseous muscles of a finger contract together, their action is to flex the MP joint, and extend both the IP joints. When one of the interossei contracts separately, it produces either ulnar deviation, or radial deviation at the MP joint.
The many fine gradations of finger movement are produced by complex interactions between the interossei, the lumbricals, and the long flexors and extensors.
The interosseous muscle on the radial side of the index finger is unusually large. It’s the first dorsal interosseous muscle. It has two heads, which arise from the first, and from the second metacarpals. The radial artery, coming round the side of the carpus, passes between its 2 heads, as we’ll see.
The first dorsal interosseous produces powerful radial deviation of the index finger. It’s one of a pair of intrinsic muscles that are strongly involved in this action, called key pinch, as in holding a key.
We’ll see the other one of the pair, adductor pollicis, when we look at the thumb muscles. Let’s move on now to look at the four lumbrical muscles.
Here are the lumbricals, one for each finger. Each lumbrical muscle arises from the side of one or both of the adjoining flexor digitorum profundus tendons.
The lumbricals pass in front of the deep transverse metacarpal ligament. Each lumbrical inserts on the radial side of the extensor mechanism, just distal to the long part of the interosseous tendon. The action of the lumbricals reinforces the action of the interossei in extending the IP joints. They also assist in radial deviation of the MP joint.