1. The intrinsic muscles of the hand are primordial, having started in the fish. The hand developed before the arm. Intrinsic muscles persisted down the animal phila and not until later did some intrinsic hand muscles develop into the long flexors and extensors (flexor profundus in reptiles and flexor sublimis and extensor brevis in mammals).

2. The interosseus and lumbricalis muscles have two separate functions, depending on whether or not the proximal finger joint is stabilized in extension by the long extensor tendon,—(1) extension of the distal two joints of the fingers together with lateral motions of the finger, and (2) flexion of the proximal finger joints.

3. A shift of the aponeurotic sleeve at the base of the finger aids in determining these movements, and a volar shift of lateral bands at the middle joint makes normal movement possible.

4. In the first half of the range of flexion of the proximal finger joints, the intrinsic muscles predominate in extending the distal two finger joints, but in the last half of flexion, the long extensors predominate.

5. Normal finger action depends on nice muscle balance, coordination, and synergy,—each of the various intrinsic and long muscles and tendons doing its part in the consecutive motion.

6. There are different types of deformities due to loss of action of various intrinsic muscles.

7. For loss of intrinsic-muscle action various methods of operative treatment are valuable in restoring muscle balance to the fingers and adduction to the thumb, and in restoring the carpal and metacarpal arches.