舌骨上筋群の比較解剖学的研究

Abstract

Food habit and predation have some effects on the development of the muscles related to mandibular movement. It seems that comparative anatomical studies of muscles and nerves in head and neck is useful. A comparative anatomical study on suprahyoid muscles of six species of mammals was made by means of macroscopy. The mammals in the present study were selected according to the individual food habit : the carnivora (racoon dogs, dogs and cats), the omnivora (pigs) and the herbivora (guinea pigs and rabbits). In a total of 39 mammals, the digastric muscle, stylohyoid muscle, mylohyoid muscle and geniohyoid muscle were observed in gross anatomically for the shape, origin, insertion, direction and innervation. The results were as follows : 1. The digastric muscle arises from the jugular process in racoon dogs, dogs, pigs, guinea pigs and rabbits, from the posterior surface of the mastoid process and the notch between the jugular process and the mastoid process in cats. It inserts into the ventral border of the mandible in all of the species. It appears as a fusiform muscle highly developed in the carnivora. It appears as a double-bellied muscle, or the posterior belly muscle is absent, in the omnivora and the herbivora. The posterior belly is innervated by the facial nerve, and the anterior belly by the mandibular nerve in all of the species. 2. The stylohyoid muscle is absent in racoon dogs and guinea pigs. It arises from the tympanohyoid in dogs and cats, from the stylohyoid in pigs, from the notch between the jugular process and the mastoid process in rabbits. It inserts into the lateral surface of the basihyoid in all of the species. It runs inside the digastric muscle in pigs, and outside the digastric muscle in dogs, cats and rabbits. It is very degenerative in dogs and highly developed in pigs and rabbits. It is innervated by the facial nerve in dogs, cats, pigs and rabbits. 3. The mylohyoid muscle appears as a single sheet in racoon dogs, dogs, cats, pigs and guinea pigs. The anterior muscle bundles arise from the ventral border of the mandible and the posterior muscle bundles from the inner surface of the mandible in racoon dogs and dogs. The both muscle bundles arise from the inner surface of the mandible in cats, pigs and guinea pigs. It appears bilayer, and the superficial layer arises from the ventral border of the mandible, the deep layer from the medial surface of the mandible in rabbits. Almost all the muscle bundles insert into the mylohyiod raphe and other bundles in the posterior portion into the ventral surface of the basihyoid. It is innervated by the mandibular nerve in all of the species. 4. The geniohyoid muscle arises from the inner surface of the mandible adjacent to the mandibular symphysis and inserts into the central portion of the anterior surface of the basihyoid in all of the species. It is a slender and triangular pyramid muscle with the origin at the apex and the insertion at the bottom in all of the specimens. It is in close contact with its counterpart on the opposite side in racoon dogs, dogs, cats and pigs, but not in guinea pigs and rabbits. It is innervated by the hypoglossal nerve in all of the species. 5. The difference of the origin of the digastric muscle and the presence of the degeneration of the stylohyoid muscle among the carnivora seem to be due to the transition of the size or to the relative position with the phylogenetic reconstruction of the stomatognathic system. 6. It seems that the difference of the development of the digastric muscle between the carnivora and the herbivora is due to the difference of their food habits. The more developed the posterior belly muscle of the digastric is, the more degenerated the stylohyoid muscle tends to be in mammals. 7. The lamination of the mylohyoid muscle and no contact with its counterpart on the opposite side in rabbits seem to make the pivoting of the right and left mandibles united