Auditory ossicle

“ The irregularly formed drum has connection with the air-containing cavity of the skull, and by a thin cartilaginous canal, the Eustachian tube, with the pharynx. The auditory ossicles are represented by only a single bone, called the columella, which most closely represents the stapes of mammals. ”

“ The mandible no longer articulates with the quadrate but forms a new articulation, by means of the con- dyle, with the glenoid cavity of the squamosal, and many modern morphologists, including the writer, are inclined to agree with H. Gadow that the quadrate has probably become the tympanic bone. In many mammals (e.g. Carnivora) this bone swells out to form the bulla tympani. The derivation of the auditory ossicles has been discussed in the section on embryology as well as in the article Ear. The presence of a chain of ossicles is peculiar to the Mammalia. ”

“ If the stricture or point of occlusion of the auditory canal is limited and composed of membranous and fibrous tissues, a good result can be usually obtained, and there is no reason why complete recovery of hearing should not take place if the labyrinth and tympanic cavity are normal. ”

“ The middle ear or tympanum is an irregular cavity in the petrous portion of the temporal bone. Its outer wall is formed by the membrana tympani or drum, an oval translucent membrane placed obliquely at the bottom of the external auditory canal. The middle ear communicates with the inner ear through the fenestra ovalis or oval window and contains the ossicles, the malleus or hammer, the incus or anvil, and the stapes or stirrup, which are arranged in a movable chain from the drum to the oval window. ”

“ The pro-otic ossifies, and either remains distinct from the epi-otic [65] and opisthotic throughout life, or unites with them only after they have fused with the adjacent bones. The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with the [190] auditory apparatus, forming the auditory ossicles [66] . Each ramus of the mandible always consists of a cartilage bone, the articular, and several membrane bones. The mandible articulates with the cranium by means of a quadrate. The ribs in Birds and some Reptiles bear uncinate processes, i.e. ”

“ The organ of hearing is without any external ear and without an air-containing external auditory meatus. The aerial vibrations do not pass, as in other mammals, from the external auditory passage to the tympanic cavity and thus to the nerve-terminations of the inner ear; but they reach the tympanic cavity by direct transmission through the bones of the skull, which possess a special structure and contain abundant air-cavities. ”

“ As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization.
The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100. ”

“ Through it the auditory nerve leaves the cranial cavity and enters the ear. The ramus vestibularis leaves through the dorsal part of the hole, the ramus cochlearis through the ventral.
The cavity of the auditory or periotic capsule communicates with the exterior by a fairly large hole, the fenestra ovalis, which lies between the opisthotic and pro-otic, and opens into a deep depression, the tympanic cavity, which is seen in a posterior view of the skull lying just external to the exoccipital. ”

“ As to the auditory apparatus itself, we see that the elaborate organ for hearing—the cochlea—has been evolved in the vertebrate phylum itself. In the lowest vertebrates the auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a simple sac forms the early stage of the auditory vesicle in Ammocœtes, according to Shipley ”

“ The vertebral centra have no epiphyses, and the odontoid process remains for a long time free from the centrum of the second vertebra. With the exception of the atlas of Echidna the cervical vertebrae are without zygapophyses. The cranial walls are smooth and rounded, and the sutures between the several bones early become completely obliterated as in birds. The mandible is a very slight structure, with no ascending ramus, and with the coronoid process (see p. 398) and angle rudimentary. The auditory ossicles show a low state of development. ”

“ Connexions can be traced to the optic tract, the higher visual centre on the mesial surface of the occipital lobe, the deep origin of the third or oculo-motor nerve as well as to the mesial and lateral fillet. The inferior pair of quadrigeminal bodies are more closely in touch with the organs of hearing, and are connected by the lateral fillet with the cochlear nucleus of the auditory nerve. ”

“ In the most perfectly organized animals—those namely which, like ourselves, possess a spinal column—the internal organs of hearing consist of two more or less complex membranous sacs (containing calcareous particles—otoliths) , which are primitively or permanently lodged in two chambers, one on each side of the cartilaginous skull. The primitive cartilaginous cranium supports and protects the base of the brain, and the auditory nerves pass from that brain into the cartilaginous chambers to reach the auditory sacs. ”

“ The organs supplied by the auditory nerve are only partly for the purpose of hearing; there is always present also an apparatus—the semicircular canals—concerned with equilibration and co-ordination of movements. Such equilibration organs are not confined to the auditory nerve, but in the water-living vertebrates are arranged segmentally along the body, forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral line organs, which has been specially developed. ”

“ Dissected to show the auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid and thyroid apparatus. All these parts are delineated on a larger scale than the rest of the skull. an, Tympanic ring; b.hy, basihyal element; hy, so-called hyoid bone; in, incus; md, bony mandible; ml, malleus; st, stapes; tp, tympanum; tr, trachea; I. (mk) , first skeletal (mandibular) arch (Meckel's cartilage) ; II. second skeletal (hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth arches (thyroid cartilage) . (From Wiedersheim's Structure of Man.) ”

“ The human ear would seem to present greater difficulties. For the essential part of the sense-organ, in this case, is the membranous labyrinth, a bag of complicated form, which lies buried in the depths of the floor of the skull, and is surrounded by dense and solid bone. Here, however, recourse to the study of development readily unravels the mystery. Shortly after the time when the olfactory organ appears as a depression of the skin on the side of the fore-part of the head, the auditory organ appears as a similar depression on the side of its back part. ”

“ The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending.
The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head ”

“ In the performance of its function the ear receives and transmits sound waves and also concentrates them upon a suitable exposure of nerve cells. It includes three parts—the external ear, the middle ear, and the internal ear.
External Ear.—The external ear consists of the part on the outside of the head called the pinna, or auricle, and the tube leading into the middle ear, called the auditory canal (Fig. 151) . The pinna by its peculiar shape aids to some extent the entrance of sound waves into the auditory canal.119 It consists chiefly of cartilage. ”

“ In the bony fishes (Teleostei) the membrane or dermal bones are still more numerous, and many of them are unrepresented in the mammalian skull, while others, which are there quite rudimentary, are very large. The chondro-cranium tends to disappear in the vault, but the base is fully ossified. Among other cartilage bones the five ossifications of the auditory capsule are seen, the pro-, epi-, opisth-, pter- and sphen-otics, all of which are found as centres of ossification in man. ”

“ The great cavity is the bulla tympani or bulb of the ear; a m is the auditory meatus or external hole of the ear. On looking into a dry skull the passage seems to be of no great depth, nor can an instrument be passed directly from the outside into the great tympanic cavity, the hindrance being a wall of bone, s, the septum which divides the bulla into two distinct chambers, the reason for which is not very clear, except that one may suppose it to be in some measure for acoustic purposes, as all animals with this development are quick of hearing. ”

“ For neither the ear nor the eye receives anything but the impulses or vibrations originated by sonorous or luminous bodies. Nevertheless, the receptive apparatus still consists of nothing but specially modified epithelial cells. In the labyrinth of the ear of the higher animals the free ends of these cells terminate in excessively delicate hairlike filaments; while, in the lower forms of auditory organ, its free surface is beset with delicate hairs like those of the surface of the body, and the transmissive nerves are connected with the bases of these hairs. ”

“ The auditory organ [8] consists of a ‘crista acustica’ (fig. 9) , in the form of a slight prominence of columnar cells on the ventral side of the anterior cerebral vesicle; to the summit of which a spherical otolith is attached by fine hairs. In the crista is a cavity containing clear fluid. The dorsal half of the otolith is pigmented: the ventral half is without pigment. ”

“ It is not the mere motion in the auditory organs which produces the sensation of sound; the nerve certainly perceives each movement in the ears, because none is possible apart from or without primary motion; only such a motion is no sound, but only a noise. What has been written in the tingling metal according to eternal laws, is transcribed or copied in the auditory nerve ”

“ If the patient before operation hears conversation at a greater distance than 12 feet he should be told that the hearing power may be reduced to this amount. If, however, there be considerable deafness, due to polypi or granulations blocking up the tympanic cavity and auditory canal, the hearing power may be improved by the operation. The ultimate hearing depends on the condition of the stapes within the fenestra ovalis: if it remains freely movable, the hearing power may be extremely good. ”

“ If the auditory canal be very small there may not be sufficient room to insert the instruments through the speculum. In such cases, if there be no middle-ear suppuration, it is wiser to leave the condition alone. If, however, suppuration exists, either the conservative treatment must be continued or the complete mastoid operation recommended.
2. Hæmorrhage, especially on curetting away the granulations, may be sufficient to prevent a view of the deeper parts. It can, however, usually be arrested quickly by plugging the auditory canal with gauze soaked in adrenalin and cocaine solution. ”

“ Such growths, provided they are pedunculated and do not abut on the tympanic membrane, can sometimes also be removed by the same method; much depends on their shape and situation. If sessile or too deeply placed, the operation may have to be completed by reflecting forward the auricle. Before terminating the operation a clear view of the tympanic membrane should always be obtained.
The meatus is finally syringed out with a 1 in 5,000 aqueous solution of biniodide of mercury and dried, a strip of sterilized gauze being inserted into the auditory canal. ”