Abstract
Fairly frequent occurrences of abnormity in the anterior belly of m. biventer mandibulae have been so far reported in the literatures. The author has found as many as 14 cases of this muscle abnormity out of 25 corpses examined in the 3 years' courses of dissection practice from 1954 to 1956 in our laboratory. Among various classification proposed by several investigators from morphological and genetical grounds there is one by Mr. Yamada for these muscle abnormities. Mr. Yamada classified them into 1) primitive 2) insertion 3) origin 4) mixed 5) double and 6) lacking types, and this classification was adopted by the present author in dealing with his cases. There were 7 cases of origin type and 5 cases of double type, followed by each one case of primitive and insertion types. Since the insertion, origin and mixed types were observed sometimes mixing with each other in the double type, it may be concluded that the primitive and lacking types were among the most infrequent occurrences. The primitive type of this muscle is claimed to be the normal occurrence in some species of monkeys, but is rarely seen in the human being. In one particular case included in the origin type groop it was observed that the muscle fibers were oriented outwardly to submaxillar triangle in contrast to all the other anomalous muscles which confined themselves to stay in the submental triangle. It seems probable that the frequent occurrence of abnormity in this muscle would be based upon the special genetic ground of m. biventer mandibulae that it develops under the control of two different nerve branches.
