Anatomical study of the incisivus labii superioris and inferioris muscles in non-human primates.

The facial muscles have significant roles for vocalization, feeding, and facial expression in both human and non-human primates. Of these, the anatomy of the incisivus labii superioris (ILS) and incisivus labii inferioris (ILI), which are considered as the accessory bundle of the orbicularis oris (OO) in humans, has rarely been documented in the literature. Our current understanding of the function of the ILS and ILI is that they probably retract the upper and lower lips. Also, there is no account of these muscles in non-human primates in the current literature. The aim of this study was to reveal the ILS and ILI in non-human primates. Five Macaca fascicularis, one Macaca fuscata, one Macaca fuscata yakui, and one Pan troglodytes were dissected. Seven formalin-fixed cadavers and one fresh cadaver were included. Both the ILS and ILI were observed in all specimens. The ILS originated from the incisive fossa of the maxilla and inserted into the OO. The mentalis (MT) and ILI arose from the incisive fossa of the mandible and inserted into the OO and the skin of the chin area. The MT and ILI in the P. troglodytes examined were thicker than in the other three non-human species, and the ILS and ILI in the three macaques were similar in shape to those of humans. The difference of these muscles may result in different functions of the lip such as during vocalization, feeding, and facial expression.