1. A study of seventeen cases of isolated paralysis of the shoulder-blade fixators was undertaken in order to determine the relative importance of each of these muscles in arm movements.
2. Tests for individual muscles are described.
3. Isolated paralysis of the trapezius, even in cases where the whole muscle is absent, does not interfere appreciably with the elevation of the arm. The disability is marked only when the subject abducts the arm with the trunk inclined forward.
4. Isolated paralysis of the serratus magnus causes far more disability than paralysis of the trapezius alone, as the subject is unable to elevate the arm above a horizontal position.
5. Paralysis of both the trapezius and the serratus magnus causes a very serious disability. The scapula completely lacks fixation, and abduction of the arm can be performed to approximately 60 degrees only.
6. Isolated paralysis of the rhomboids or the pectoralis minor was not present in any of the cases studied. The exact effect of the loss of one of these muscles, therefore, could not be determined. In cases with weak rhomboid muscles, resistive adduction of the arm proves to be rather difficult, as the teres major pulls the scapula toward the arm instead of pulling the arm toward the scapula.
7. Isolated paralysis of the levator scapulae was not found in any of the cases studied. When the upper trapezius is absent and the rhomboids weak, the subject has no difficulty in elevating the shoulder girdle against resistance. It seems, therefore, reasonable to assume that in normal subjects, too, the levator scapulae is the most important muscle for elevation of the shoulder girdle.
8. The etiology could not be determined with certainty in all cases, but five groups were recognized: injury to the long thoracic nerve, injury to the spinal accessory nerve, congenital defects, muscle dystrophy, and poliomyelitis.