Intrinsic auricular muscles and auricular form.

Plical folding of the cartilagenous ear plate is largely determined by the insertions of four extrinsic and seven intrinsic auricular muscles. Dissection of ten external auricles from anencephalic fetuses of six or more months' gestation was done. The transversus and obliquus auricular muscles found in association with the antihelical fold were evaluated. Four auricles were of normal form and had the transversus and obliquus auricular muscles with typical insertions in the cartilage. Six of the ten auricles had a simplified form, with no antihelical folding in five and minimal development in one. Fibers from neither the transversus nor the obliquus auricular muscles were identified in these six ears. The results of these dissections strongly suggest that antihelical folding is determined by the presence of functioning transversus and obliquus auriculae muscles. Similarly it is proposed that normal folding of the auricular cartilage is dependent on the presence of functioning auricular muscles. In evaluation of the child with absence of normal ear form, these findings suggest that the unusual shape is due to abnormal placement and/or function of the intrinsic and extrinsic auricular muscles. These changes may be a subtle sign of neuromuscular weakness, abnormal muscle insertion or absence of muscle.