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In 1960, the functional matrix hypothesis was first articulated by Moss. Although many investigators agreed that there were genetic influences on craniofacial growth, the functional demands of matrices are more significant in determining craniofacial morphology during growth. Therefore, during postnatal growth, an intimate functional relationship exits between the jaw muscles and the bones to which they are attached. The most recent findings suggest that the morphological changes can be observed not only in areas adjacent to muscle insertion but also in growth sites affecting the rotational growth pattern of the cranium. If this is true, the reduced functional demands on the masticatory muscles may cause some changes of craniofacial growth. To test this hypothesis, a longitudinal study by means of cephalometric analysis to observe the changed masticatory function induced by a diet of physically soft food on the craniofacial morphology in the growing rabbits was conducted. One group of rabbits was fed with a soft diet, while another group received a standard laboratory diet and served as the control group. These animals had been weaned at the start of the experiment (1 month of age), and the experimental period last for 6 months. During that time, all rabbits were weighed every month, and lateral cephalograms were taken with the skull fixed in a specially constructed cephalostat. The procedure of taking a cephalogram entails the sacrifice of the subject. Thus five rabbits in each group were sacrificed each experimental period. The radiographic images were copied on photographic paper, magnified by two and the cephalometric analyses were performed. The results are as follows: 1. There were no statistical differences between groups in any of the test periods for body weights. 2. The results of cephalometric analysis showed that no difference in overall skull shape could be found between the two groups, but a more anterior superior growth rotation of the upper jaw occurred in the rabbits fed with soft diet compared to the rabbits fed with a standard laboratory diet. Thus, the skull became more erect position in the soft diet group. 3. In the soft diet group, the angular process of the mandible was less convex. That was a clockwise growth pattern. In addition, there was a thin and flat shape of mandibular body in compared to control group.