VII.-Sketches of the Comparative Anatomy of the Organ of Hearing, founded chiefly on the Ear of the SQUALUS. By THOMAS BUCHANAN, C. M. Member of the Wernerian Natural History Society, Author of the • Illustrations of Acoustic Surgery,' &c. &c. (Read 19th November 1825.) THE organ of hearing in the Shark tribe varies considerably from that of the human subject. We find neither ossicula auditus, tympanum, eustachian tube, nor cochlea,-—but, as if to compensate for the want of parts so essentially necessary to the perfect ear, the semicircular canals are of almost incredible extent. The cranium being composed of semitransparent cartilaginous substance, of a beautiful cærulean hue, the whole of the organ can be distinctly seen, by merely removing the cuticle, and some of the surrounding parts. When the cranium is divested of the common integuments, a considerable depression is seen in the coronal and posterior portion, which, in the living subject, is occupied by a kind of spongy, elastic, cellular membrane. This depression is of a heart-like figure, the apex pointing forwards, with a roundish process in the posterior part, which causes a partial division of this portion of the cavity. a In the older fish, the intermediate space in the depression between the cuticle and the cranium is chiefly filled with a transparent gelatinous substance, which can easily be seen and felt in the recent subject, by passing the finger backwards over the depression, when the posterior part will become tumid like a bladder. In the posterior and rather inferior and external parts of the depression are four foramina, two of which are situate on each side of the posterior process. The foramen next to the process is large, and of a circular figure, and, in the recent subject, covered with a membrane, the plane of which forms an angle of nearly 45°, with a line through the centre of the cranium, and about the same angle from the perpendicular. This membrane may with propriety be termed the Membrana labyrinthi. The spongy, elastic, membranous substance is attached to the external side of the membrane; but, in the Skate, there is a considerable space between it and the substance. This large foramen leads into the vestibule and posterior semicircular canal, and may be termed the Foramen rotundum, from its circular figure. Close to, and outside of this foramen, is a very oblong aperture, through which the tube of the ear, in the young subject, enters the vestibule, and which, from its extreme oblong figure, may be termed the Foramen oblongatum. In the young subject of the species Squalus canus, the meatus auditorius externus is situate on the superior and posterior or coronal surface of the head, is small, and much contracted, and seldom admits of any substance larger than a strong bristle. The tube is formed of tough, elastic, membranous substance, and runs outwards or laterad, forwards or antinead, and downwards or basilad, a few lines, more or less, according to the size of the animal, until it reaches within short space of a membrane stretched across the tube, where it enlarges to nearly twice the size of the other a parts of the tube. This membrane may with propriety be termed the membrana vestibuli. The tube then describes an angle by running downwards, backwards, and a little outwards, until it reaches the foramen oblongatum, to the edge of which it is attached, in some subjects more closely than in others. In young fish of the Squalus canus, the meatus and auditory tube are more easily found than in the adult, or in some of the other species. In old fish the meatus is generally almost obliterated; the tube and membrana vestibuli are seen, but indurated and enlarged, so as to be scarcely recognisable by the above description. This alteration of the parts may perhaps be the consequence of disease, or the effect of old age. In the ear of the adult of the Balæna mysticetus, I have frequently found the stapes so firmly attached to the foramen ovale, that the union resembled ossification, and required considerable efforts to separate the bone from the foramen ;-the probable cause of this adhesion will be pointed out, when describing the ear of that animal, so that one fact may, if possible, throw , light on another. The following are the dimensions of the parts in a preparation of the Squalus canus, three feet in length. Distance between the orifices of the meatus audito rius externus, vestibuli, 2} lines. däth of a line ¢do. 1 line. 1} lines. In the species S. borealis, or Greenland Shark, the magnitude of the semicircular canals is such, as to surpass any idea which may be formed of the parts from the dissection of the organ in the human subject. The superior size of the parts may be estimated from the dimensions of a cast of the left ear of that animal now before me, which I took a few hours after it was killed. * The entrance of the meatus internus is about three lines in diameter, and situate in the inferior part of the organ. It runs downwards, and a little outwards, about two lines and a half; then contracts suddenly, and runs horizontally, outwards, nearly a line, when it enters the vestibule, parallel with the floor of that cavity. The vestibule is large, of an irregular triangular figure; the apex of which may be said to be directed towards the brain, while the base runs horizontally outwards and backwards. The circumference of the greatest diameter of the vestibule is twenty-five lines, and the height of the cavity, from the highest to the lowest points, twenty-two lines, On the inside of the superior part of the external angle of the vestibule, is a longitudinal ridge, which, in the sketch of the cast, is seen as a depression ; and on the floor there is a correspondent ridge on the opposite side, that runs upwards on the parietes next to the brain, until it arrives at the top of the cavity, where it unites, and forms a septum, which separates the foramen oblongatum from the foramen rotundum. The floor of the vestibule is more tough and hardened than any other part of the labyrinth, particularly that which is directly under the sabulous body, where it has a white, scaly, opaque appearance, approaching towards ossification. The whole of the vestibule is lined with a reflection of the dura mater, which is closely attached to its parietes, where it is considerably less dense than in the inside of the cranium, • See Plate I. figs. 1. and 2., where the parts are shewn of the natural size. а and still less so in the cartilaginous semicircular canals, where it is almost pellucid in the adult fish, and beautifully transparent in the young. There are three semicircular canals which arise from and communicate with the vestibule, similar to those of the human subject; and from their relative situations to that cavity, may be termed the Posterior, Anterior, and External or Horizontal. The extremities of these canals, adjoining to the vestibule, are considerably enlarged, so as to have a similar appearance to that in the human subject termed the Ampullæ, whereas the proper ampullæ of these cartilaginous canals are situate at a considerable distance from the vestibule, and are of an oblong figure. The posterior canal is the largest, and measures fifty-three. lines, or nearly five inches and a half in length, and two lines by two and three quarters in diameter. It runs in a longitudinal direction from and to the vestibule, and, when viewed internally, has the appearance of a circle attached to the ampulla of the external canal. The circumference of the ampulla is sixteen lines, and the average circumference of the other parts of the tube eight lines and three quarters. The external canal is the next in size, and measures thirty-six lines in length, and seven lines in circumference. The ampulla of this canal, at its greatest circumference, measures only ten lines and a half. The canal runs in a diagonal direction, the one extremity arising from the superior and posterior parts of the vestibule, and the other from a globular space communicating with the inferior and anterior parts of that cavity. The anterior canal, although the smallest of the three, is yet of considerable size, measuring thirty-one lines in length, and from five to seven lines in circumference. These canals are partially flattened, so that if any of them were divided at a little distance from the ampulla, а |