A cinefluorographic study of mandibular movement during feeding in the rat (Rattus norvegicus)

The anatomy of the masticatory apparatus, and particularly of the mandibular joints, has led to the view that mandibular movement in the Rodentia is predominantly propalinal, or forwards and backwards in direction. As part of an investigation into the mechanism of function of the mandibular joints in these animals, the feeding behaviour of "August" strain and "Wistar" rats was examined by cinephotography and cinefluorography. The rats were trained to feed on barium sulphate impregnated biscuit and animal cake and to drink radio-opaque liquids. Cinefluorographic recordings of ingestion, mastication, deglutition and of drinking were taken in both the lateral and dorso-ventral projections.
Analysis of the recordings has shown a fundamental separation of ingestive and masticatory activity in the rat, which can be attributed to the morphology of the jaws and particularly to the disparity in the lengths of the mandibular and maxillary diastemas. To bring the incisor teeth into occlusion for ingestion, the mandible is brought forward through the rest position and the condyle into articulation with the anterior part of the fossa. In mastication the condyle is moved backwards to bring the molar teeth into occlusion and the condyle into articulation with the posterior articular facet on the fossa. Once the mandible has been moved into the appropriate position for either ingestion or mastication and deglutition, the movements involved in the separation or chewing of the food are cyclical with combined horizontal and transverse movements as well as the fundamental vertical movement acting to open and close the mouth. The basic movement of ingestion carries the mandibular incisors upwards and forwards across the lingual surfaces of the maxillary incisors, so separating the bite. The grinding stroke of mastication is a horizontal movement carrying the mandibular molars anteriorly across the maxillary.