The lateral pterygoid muscle, is usually considered as responsible for the open mouth mechanism. Nevertheless, its function as well as disk displacement remains unclear and controversial. Anatomical dissection was carried out to analyze temporomandibular joint and lateral pterygoid muscle. Videofluoroscopic images of healthy subjects were made to observe the condyle during rest, centric occlusion, and non-resisted mouth opening. This work describes two articular joint cavities on each side. The inferior cavity allows rotation from occlusion until rest functional position. In the superior cavity the translation movement takes place entailing anterior displacement and depression of the mandible condyle. The lateral pterygoyd muscle may be considered as a biceps one. Its two heads act in synergic and complementary roles. Three basic directional muscular fibers in the two heads can be identified as possibly acting in a sequential and coordinated function to obtain the mandible depression. The superior head of the lateral pterygoid muscle pulls the mandibular condyle up, to adjust and impact it so that the condyle crest sits inside the articular disk concavity forming a disc/mandibular unit. Next, the medial muscular fibers from both heads pulls the disk and condyle unit forward and in sequence the inferior fascicle pulls the unit down causing the mouth to open. The disk-mandibular impaction must remain as a unit until the mandible return to the rest position.