The malaris muscle: its morphological significance for sustaining the intraorbital structures.

The orbicularis oculi muscle, an important mimetic muscle, was investigated to ascertain its anatomical relation to facial aging-especially its orbital part (Oo). Previous studies of the distinct muscle bundles frequently found inferior to the Oo have provided various definitions, including that of the malaris muscle. This study aimed to examine these muscle bundles and clarify their function in facial aging. Twelve heads of Japanese cadavers (average age: 82.5 years old) were dissected to observe the muscles, focusing in particular on those in the periorbital region. Six specimens were further dissected from the inner surfaces to examine the patterns of facial nerve branches under the operating microscope. Histological examinations of two head halves were carried out to investigate the relationship between the muscle bundles and the intraorbital structures. Muscle bundles consisting of lateral, medial, and U-shaped suspending bundles were observed in the region inferior to the Oo. Lateral and suspending bundles were found in all specimens, while the medial bundles were noted in only 9 of 22 specimens. Some branches of the facial nerve penetrated through the lateral, medial, and suspending bundles. The relationship between the suspending bundles and the protruding orbital fat was assessed. The muscle bundles found in this study were regarded as the malaris muscle-a transitional muscle between the superficial and deep facial layers. The suspending bundle may play a role in sustaining the intraorbital structures.