Functional Anatomy of the Masticatory Apparatus in the Rabbit (Oryctolagus Cuniculus L.)

Abstract

Jaw muscle activity and mandibular movements were studied in the domestic rabbit by quantitative electromyography and cineradiography. Mandibular movement profile and occlusal interaction depend on the consistency of the food masticated. Minimal vertical and maximal transverse jaw excursions combined with initial edge-to-edge apposition of cheek tooth ridges, followed by an oblique antero-medial shearing movement of the lower teeth into intercuspation characterize hay mastication. This movement is followed by a purely transverse grinding stroke guided by the interlocked ridges and valleys of the teeth. In carrot mastication a vertical, crushing movement predominates. In laboratory pellets, the movement profile is intermediate; occlusal movement is usually confined to a transverse grinding stroke. During chewing jaw closure and deflection toward the working side are caused by simultaneous action of balancing side superficial masseter and medial pterygoid and working side deep masseter and temporalis. The lingually directed power stroke is produced by working side superficial masseter and medial pterygoid and balancing side deep masseter and temporalis. Firing levels are higher on the working than on the balancing side and decrease from hay to pellets to carrot. Transverse muscle force components, generated mainly by the pterygoids also decrease in the series hay-pellets-carrot. The ratio between forces on working and balancing sides allows the balancing side joint to act as a fulcrum; the working side joint remains unloaded. During biting joints are loaded symmetrically. The unique cheek tooth structure allows lagomorphs to apply various combinations of crushing, shearing and grinding forces to their food. By combining strong protractive and transverse muscular forces they can adjust bite force direction. It is argued that this versatility enables them to utilize a wide variety of vegetable matter as food and remain generalized and adapted to different niches.