Muscle-fibre architecture of the rat medial pterygoid muscle.

The detailed fibre architecture of the rat medial pterygoid muscle, including the courses and attachment points of muscle fibres, was investigated histologically in 10 micron thick serial sections in the horizontal, coronal and parasagittal planes. Four extramuscular tendinous sheets (external aponeuroses) and four intramuscular tendinous sheets (internal aponeuroses) were found. Three of the internal aponeuroses were arranged parallel to the rostro-caudal axis; the other was oblique to the rostro-caudal axis. Muscle fibres were located between internal aponeuroses, between external aponeuroses, between internal and external aponeuroses, between an internal aponeurosis and the periosteum, and between an external aponeurosis and the periosteum. The courses of muscle fibres were divided into three main groups: vertical, rostro-caudal and medio-lateral. Eight compartments were distinguished. The multiple movement of the medial pterygoid muscle seemed to be due to this compartmentalization. These findings suggested that the rat medial pterygoid muscle, like the rat masseter muscle, is mainly composed of multipennate muscles and compartmentalized into many muscle-fibre bundles running in different directions.