The 'bone within a bone'.
D001846:Bone Development; D001847:Bone Diseases / Q000000981:diagnostic imaging*; D001842:Bone and Bones / Q000000981:diagnostic imaging*; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D011859:Radiography