諸種脊椎動物聴器の計測的研究

抄録

The author measured the diameter, the length and the capacity of external auditory meatusand middle ear cavity in 34 different species of vertebrates including 2 amphibians, 4 reptiles, 6birds, and 22 mammals, to study the mutual relationship of each measurement and also studied the morphology of the middle ear cavities.The conclusions were as follows:-
1). The length of the external auditory meatus was always in proportion to the size of theskulls, but not to the breadth. The larger animals had longer meatus. The ratio of the length tobreadth of meatus was 11.4 in ox, 13.7 in pig, but in small animals like bat or rat, was nearly 1.0.
In animals which behave in water, for example, otter, sea-lion and fur-seal, the ratios wereremarkably large. The partially closed meatus observed in whale was an extreme case.
2). The external auditory meatus is essential for fine perception of air conduction sound.Moreover, the resonance effect of the meatus which is determined by the length of it may profitthe hearing acuity of the animal, when awared of the close relationship between tbe resonanceeffect and animal's cry.
3). The capacity of tympanic cavity was always larger than those of external meatus, and theratio dividing the former by the latter was very much constant in each mammal, for example, 3.0_??_3.4 in dogs. and 3.7_??_4.1 in cats.
4). In general, the capacity of the tympanic cavity was proportional to the size of the head, mainly due to the capacity of the tympanic bulla. On the other hand, the mesotympanum contain-ing the tympanic membrane, ossicles and windows showed a little variation.I have observed also this tendency among the different mammals. The ratio of tympanic cavityper 1mm2 of ear drum was 4 in rat, 6 in mole and 435 in ox.
5). The tympanic cavity of bird was uniform in shape as well as external meatus, exceptsome variation in shape and size of air cells fully developed in the skull.
6). The form of accessory air spaces of vertebrates were classified into several types accordingto the"order"or "family", such as single bulla with or without bony septum (rodentia, chiro-ptera, some uugulates and cetacea), and cellular structure developed in the bulla (ox, pig andhorse), in the mastoid process (some primates), or in the basis of the skull (insectivora).
7). But certain irregularities of the structures of the auditory organs were noted, because ofthe adaptation to the circumstances. For example, Barary sheep, classified in bovidae, had a singlebulla and large epitympanum with large Schrapnell's membrane as seen in cervidae.
8). Accessory air spaces are thought to have significance in four points, that is, making the tympanic membrane easily vibrate by the low frequency sound waves, reducing the weight of theskull, and the effect of reservoir action and acoustic dissipation. The resonance or insullation effect are doubtful.
9). The size of the air sinuses in the skull has a tendency to change according to the size of the skull. Therefore we must consider the relation of the both when the air sinuses are discussed.