COMPARARICE ANATOMICAL STUDY ON THE MIDDLE EAR MUSCLES OF VERTEBRATES

Abstract

Author studied comparative anatomy of the middle ear musclature of the vertebrates, i. e. bufo, snake, cock, mogera, guinea pig, rabbit, dog, cat, fur seal, sheep, pig, horse, chimpanzee and man. The conclusions are as follows: 1. Bufo (Amphibia) and cock (Aves) have only one middle ear muscle. Snake (Reptilis), whose middle ear is made as if it percepts only the bone conduction sound, has no middle ear muscle. Mammals have two middle ear muscles. But the degernerated tensor tympani muscle of mogera contains no muscle fibres and forms a ligament. It is thought that this structure is for the specific condition of life, that is under the ground. 2. In Mammals the stapedial muscle is innervated by the facial nerve and the tensor tympani muscle is innervated by the trigeminal nerve. The middle ear muscle of cock is innervated by facial nerve, the opercula muscle of bufo seems to be innervated by the branch of the cervical nerve. 3. In Mammals, tensor tympani muscles show the various forms. Tensor tympani muscles of rabbit, sheep, pig and horse are composed of two portions, pars tubaria and pars petrosa. Those of guinea pig, dog, cat and fur seal are composed of only one portion, pars petrosa. In chimpanzee and man the tensor tympani muscle is composed only of pars tubaria. 4. In bufo and cock, the arrangement of fibres of the middle ear muscle is nearly parallel to the longitudinal axis of the muscle itself. In mammals, it is characteristic that all the stapedial muscles are penniform. But in mammals the fibre arrangement of the tensor tympani muscle shows various directions. 5. Bufo and cock (lower vertebrates) have only one middle ear muscle and snake has no middle ear muscle. But higher vertebrtes (mammals) have two middle ear muscles, each of which inserts stapes and hammer. In chimpanzee and man which are most evoluted in mammals, the tensor tym-pani muscle reveals the typical penniform muscle composed only of pars tubaria.