The normal anatomical relations and the physiological action of the supraspinatus muscle in the movement of abduction are such that frequent and continued action may produce a profound change in the character of the muscle.

The change in the muscle consists essentially of an alteration in the character of its lateral portion,—from fleshy fibers ending in a short tendon to a wide aponeurosis of fibrous tissue which blends with the infraspinatus.

Once the muscle has been weakened by this alteration, a series of other changes become imminent possibilities. These include calcification, splitting or rupture of the tendon, separation of the sheet from the greater tuberosity, and the establishment of free communication between the subacromial bursa and the joint cavity.

Associated and subsequent changes occur in the greater tuberosity, in the intertubercular sulcus, in the articular cartilage, in the tendon of the long head of the biceps, and in the walk of the joint cavity.

The long head of the biceps (capsular portion of the tendon) is at first flattened and frayed. Eventual disappearance of this portion is not uncommon.

The purpose of this communication is to emphasize the fact that these results are not caused by sudden acute ruptures of the supraspinatus, but are preceded by a long preliminary wearing effect on the tendon of the supraspinatus. On this account the age of the patient and the type of occupation are important etiological factors.

From all figures available it would appear that about 20 per cent. of all adult shoulders show some degree of change in the supraspinatus. Further, about one-quarter of this 20 per cent. will suffer some degree of rupture or splitting. On this basis, the condition as described constitutes the most common form of shoulder disability. Comparison of these figures with the number of cases reported suggests that many of these shoulders are undiagnosed and untreated.