The course and insertion pattern of muscle fibres in the philtrum of the human upper lip were studied in seven post-mortem specimens using serial histological sections and a reconstruction method using sheets of Plexiglas. The fibres of the musculus orbicularis oris entering the upper lip from one side were observed to decussate in the midline and proceed to insert into the skin of the opposite side lateral to the philtral groove. The philtral ridges appeared to represent the medial borders of bilateral muscle insertion zones of the lip in which the fibres of the m. orbicularis oris raised the level of the skin by splaying out and inserting into it. Contributions to philtral form also came from the musculus levator labii superioris as it descended as far medially as the philtral ridge to insert into the vermilion border lateral to the median groove, and from fibres of the musculus nasalis which inserted into the philtral ridges superiorly. The philtral groove corresponded to the more compact median decussation of the orbicularis oris where lip thickness was also reduced by the relative absence of muscle fibre insertions into the skin. The displacment of the vermilion border giving rise to Cupid's bow appeared to result from the lifting action of the m. levator labii superioris lateral to the median groove in conjuction with a depressor action by the m. orbicularis oris on the median tubercle.