The facial muscles of the rhesus monkey (Macaca mulatta)
The study of non-human primate anatomy developed in the late nineteenth century, following the work of Darwin on the principles of expression. These early efforts emphasized the evolutionary approach to the development of muscle systems. The major descriptive work on the facial muscles of the rhesus monkey (Macaca mulatta) was done in the 1920's and 1930's by Ernst Huber and G. S. Lightoller. Huber continued the phylogenetic approach of earlier anatomists. He produced major works on the evolution of facial muscles, as well as the standard descriptive anatomy of the rhesus facial muscles. Lightoller took a functional viewpoint, based on his studies of human facial muscles. These two approaches produced very different results, both in terms of nomenclature and substantive findings. Dissections were made of a juvenile and an adult male rhesus monkey. The results were compared with those reported by Huber and Lightoller. Some problems in interpretation arose because of the incompleteness of Lightoller's data. The research results generally support Huber's views. There is evidence that Lightoller was correct in ascribing the origin of the m. platysma to two separate matrices. It is apparent that the mm. caninus and triangularis are a single muscle, as Lightoller stated. The remaining disagreements resolved in favor of Huber's position. This was particularly true of Huber's claim that the incisivi muscles do not exist in the rhesus monkey. The results indicate the dangers of looking for structures homologous to those found in man when such structures may not have differentiated in the species under examination. Neither the phylogenetic nor the functional approach is intrinsically better for the study of facial muscles. Both can be used, with allowances for the drawbacks in each. Current research is applying the techniques of electromyography to the analysis of facial muscles and their relationship to facial expression.