Vestiges of vibrissal capsular muscles exist in the human upper lip

In nonhuman mammals, sinus hairs grow around the nostrils and serve for tactile sensation. The hair follicles of sinus hairs are rooted in vibrissal capsular muscles connected to the underlying, deeper orbicularis oris, thereby enabling the voluntary movement of sinus hairs. These vibrissal capsular muscles as well as the sinus hairs are believed to have been lost during human evolution, and no previous reports can be found on the existence of vestiges of sinus hairs in humans. Our study, however, has now verified the existence of vestigial muscles of vibrissae in the human upper lip. Using conventional histological techniques, microscopic observations were made on specimens obtained from the upper lips of human adult cadavers. In 35% of these individuals, several striated muscle fascicles diverging from the underlying orbicularis oris and reaching the hair follicles were observed. Histological findings of the vibrissal capsular muscle complex suggest that these fascicles are a vestigial remnant of the sinus hair muscle.