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A new vision on audition: a comparative study of the apparatus concerned with equilibrium and hearing. K Shyam KishoreDepartment of Anatomy, Seth G. S. Medical College and K. E. M. Hospital, Parel, Mumbai-400 012, India. , India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.918 Keywords: Animal, Comparative Study, Ear, anatomy &histology,Equilibrium, physiology,Facial Nerve, anatomy &histology,physiology,Hearing, physiology,Human, Phylogeny, Vertebrates, Vestibulocochlear Nerve, anatomy &histology,physiology,Vibration,
From fishes to man, sound reception and balance are combined in one sensory organ and in a single cranial nerve; nobody knows why this should be, or what connection (if any) sound reception has with balance. It is a mystery.[1] The nerves supplying branchial arches are mandibular, facial, glossopharyngeal and vago-accessory complex. Their foramina of exit from the cranial fossae, viz. foramen ovale, internal acoustic meatus and jugular foramen are in a single para-sagittal plane. The branchiomotor fibres of all but facial, traverse from their foramina caudally, lateral to the primitive dorsal aorta.[2] The nerves then cross antero-medially, superficial to the dorsal aorta (forming internal carotid artery for the first three arches) but deep to external carotid artery (a ventral branch of the third aortic arch), towards their corresponding branchial arches conforming to the curvature of the arch. The facial nerve is a prominent exception. It initially courses laterally along with the eighth nerve, into the middle ear. Here, the nerve takes an abrupt posterior bend (genu).[3] The geniculate ganglion is lodged here. Finally the nerve traverses inferiorly to emerge out of the stylomastoid foramen and then courses antero-medially through the substance of the parotid gland, thereby coursing laterally to both the internal and external carotid arteries.
A highly developed sensory organ of a type unknown to land dwellers is the lateral line organ seen only in fishes and larval form of amphibians. They respond to water vibrations or currents and aid the aquatic in locomotion through the water where there are no landmarks. These are ectodermal placodal in origin. The cranial end of the body makes first contact with the environment. As a result, special sense organs and the associated nervous structures are most advantageously situated here. Hence, in all vertebrates, there is a concentration of nervous tissue at the cranial end of the neural tube – a process called encephalisation. It is therefore understandable that the specialised organ for equilibrium in higher animals (also ectodermal placodal in origin), will be formed at the cranial end of the lateral line organ of fishes.
The first thoughts of the ear could be misleading since we associate it with the ornamental pinna of the mammalian ear; or perhaps, to the ossicles lying behind the ear drum. These things are entirely lacking in fishes. The basic structure of the ear in all vertebrates is the internal ear buried in the otic capsule. Before considering the hearing function, a basic function of the ear which is unchanged from fish to man is the sense of equilibrium. There are organs similar to utricle and saccule in all vertebrates. In higher fishes, semicircular canals are also seen. The receptors of these organs are similar to those of the lateral line organs. The nerves for both these organs are closely associated emerging from the hindbrain. Hearing in fishes is probably carried out by the utricle and/or saccule. The macula of utricle and saccule in humans respond to sound vibrations of low frequency.[4] The cochlea makes its appearance only in aves and mammals.
Higher fishes have an air bladder as a resonating chamber between the external and internal ears. This sets up a contrast of media between the external ear filled with water in the aquatic fish and the endolymph in the internal ear. The ossicular chain of higher animals that are derived from branchial arches are obviously missing – for the gills in fishes are responsible for respiration! This air bladder is an obvious pharyngeal diverticulum – a pattern retained up to humans as the auditory tube and middle ear cavity. The faint air waves of sound vibrations in terrestrials can have no effect in setting up endolymphatic vibrations in the internal ear. The amphibians, reptiles and aves show the stapes (developing from the second branchial arch). The pharyngeal cleft becomes the external ear and the pharyngeal pouch forms the middle ear. The membrane in between forms the tympanic membrane. The bones of the first branchial arch that lie close to the ear drum form the jaw-joint and responsible for feeding! The cartilage of the first arch (Meckel’s cartilage) forming the jaw-joint in lower animals is replaced by a new temporo-mandibular joint, where the articular process of the mandible is developed in membrane. This articular end, like clavicle, is therefore covered by fibrocartilage and not the articular hyaline cartilage. The dorsal end of this Meckel’s cartilage articulates with stapes to form malleus and incus in mammals. This further enhances amplification of sound by a factor of 1.3.[5] Thus breathing aids have become feeding aids and, finally, hearing aids.[6]
The facial and the vestibulocochlear nerve emerge at the pontomedullary junction. Both these nerves traverse laterally through the internal acoustic meatus, where the latter ends by innervating the internal ear. This passage is in the sub-arachnoid space. A dural sleeve covers the motor and sensory roots of the facial nerve. The nerve pierces the dura at the fundus of the internal acoustic meatus where the two roots fuse. Sensory and motor roots of any nerve unite at the site where the nerves pierce dura, which is also the site of location of the pseudounipolar ganglia of the sensory component. Thus all sensory ganglia (not just the trigeminal ganglia in the Meckel’s cave) are partly bathed in the cerebrospinal fluid and partly extradural. The laws of neurobiotaxis[7] cancel each other here – the “urge-to-centralise”, drawing the ganglia intradural, negates the “move-towards-the source-of-stimulus”, pulling the ganglia extradural. The diverticulum from the pharynx (air bladder) causes the posterior bending of the facial nerve. This nerve, in a newborn, emerges laterally from behind the middle and external ear. The mastoid process is not developed and the nerve is subcutaneous and liable to be injured.[8] The pull of sternocleidomastoid causes the development of the mastoid portion of the petro-mastoid part of temporal bone. This makes the nerve quite deep and secure, and also accounts for the inferior course of the facial nerve. It is peculiar that the internal ear in the petrous part of temporal bone in a newborn is of full adult size at birth, and the mastoid process and the bony part of external ear is not developed at all. In conclusion, let me quote from the preface of the first edition of the textbook of Anatomy: Regional and Applied by R. J. Last. “I sincerely hope that your reading of the preceding article may not only prove profitable to you but will stimulate your permanent interest in a fascinating subject, much of which is still not fully understood.”
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