A Comparative Study of Facial Growth in Homo and Maccrca

Sections were prepared throughout all areas of the various facial bones in young, growing Rhesus monkeys. The detailed distribution of resorptive and depository surfaces and the distribution of endosteal and periosteal bone tissue types were determined. From this information, the sequence of remodeling changes associated with the growth of the facial skeleton was then interpreted. This study is a sequal to previous reports in which growth and remodeling processes in the human face were described using similar procedures. In the present report, growth changes in the monkey and human facial skeleton are compared and contrasted. The general plan of facial growth is similar in both species, but major differences exist in the area of the muzzle. The maxillary arch in the monkey is entirely depository in nature, and it grows in a forward and downward direction as the maxillary tuberosity simultaneously grows backward. In the human, the forward part of the maxillary arch is resorptive in character. This contrasting growth factor results in a downward but not forward movement of this area. The result is decreased prognathism. Other differences in growth pattern exist in the forehead, malar, chin, and orbit. The developmental and phylogenetic basis for the upright human face is discussed and evaluated. The purpose of this study is to compare and contrast the detailed sequence of remodeling changes that take place during the growth of the human and monkey facial skeleton. In previous studies, the variety of growth processes occurring in the bones of the human face, including the frontal bone, the maxilla, nasal, zygomatic, and the mandible have been described (Enlow and Harris, '64; Enlow and Bang, '65; Enlow, '66). A history and a survey of the literature dealing with facial growth was also presented. In the present report, growth changes taking place in corresponding facial bones of Macaca mulatta are described and illustrated. Differences and similarities in the sequence of growth and remodeling processes between these two primates are then discussed and evaluated. MATERIALS AND METHODS Complete, well preserved, dried skulls from 11 normal Rhesus monkeys having mixed deciduous and permanent dentition were selected for study. Only young, rapidly growing individuals were used since sequential growth changes were determined by a n interpretation of remodeling processes associated directly with skeletal growth (see below). Multiple ground sections were prepared from the right side of each skull throughout all areas of the frontal bone, the maxilla, premaxilla, the nasal, and the zygoma. Serial stained sections prepared by decalcification and microtome sectioning were made throughout the entire right half of each mandible. In all preparations, the whole thickness of the cortex was sectioned and studied in order to analyze internal as well as external surface remodeling changes. During bone growth, a companion process of structural remodeling takes place that functions to continuously maintain the configuration of the bone as a whole (Enlow, '63). As the bone increases in overall size, its various parts and areas become successively repositioned into new locations. This continuous process of relocation requires constant remodeling adjustments in the regional shape and dimensions of all the local parts of the bone. A bone does not increase in size simply by new bone deposition on all outer surfaces. Rather, processes of bone removal and deposition on different periosteal and endosteal surfaces throughout the entire growing bone bring about generalized inAM, J. PHYS. ANTHROP., 24: 293-308. 293