Rupture of the tendons of the short rotator cuff of the shoulder joint, notably the supraspinatus tendon, is important because of the frequency of its occurrence and because of the disability that may follow. The whole subject of rupture of the short rotator tendons of the shoulder has been reviewed at length by one of us (C. L. Wilson) in another publication.1 The purpose of this paper is to present a gross and microscopic study of the supraspinatus tendon at various ages made especially with a view to amplifying the present rather scanty information regarding the pathologic changes in the tendon leading to rupture. McMaster,2 from experiments on rabbits, has shown that when a system composed of a muscle and its tendon, together with their bony attachments, is stretched to the breaking point by a force exerted through the bones the break occurs anywhere but in the tendon,