ANATOMICAL CONDITIONS OF STRENGTHENING OF THE ACOUSTIC WAVE IN THE MIDDLE EAR IN SELECTED SPECIES OF MAMMALS

Anna Charuta, Jarosław Wysocki, Urszula Wieczorek
Vertebrates Morphology Department of University of Humanities and Sciences in Siedlce, Poland

ABSTRACT

The aim of the research was to analyze anatomical bases for the effect of sound strengthening. Lenghts of the arm of the lever created by the auditory bones were also measured. The research was conducted on the temporal bones of 9 species of mammals: humans, Macaques, dogs, cats, American Minks, cattle, guinea pigs and rats. Twenty bones of each species were observed: 10 males and 10 females. The calculations were made with the use of a computer programme – Multi Scan. The research showed that carnivore (cats, Minks and dogs) had the best anatomical conditions for sound strengthening in the middle ear.

Key words: middle ear, measurements, strengthening of acoustic wave, mammals.

INTRODUCTION

Vestibulocochlear organ is present only in vertebrates [4,5]. The middle ear is found only in land vertebrates [4,5]. Well-formed external ear, the tympanic cavity with three auditory bones and the cochlea, are the structures observed only in mammals. Mammals have three auditory bones, apart from phylogenetically older stapes, there are also: the malleus formed from the joint bone and the incus, which is a homolog to the quadrate bone of reptiles [4]. The stapes of monotremes and marsupials do not form arms but they have a shape of a uniform plate, column-like shaped [4,5]. The malleus of Marsupials Has a significant length, its shape is characterised by a fracture of the axis in the cervix. The malleus of the type is similar to a hook [1]. The auditory bones of the higher mammals show significant interspecies variability while maintaining a uniform model of functioning. The stapes show the smallest changeability as far as the shape is concerned [2]. The auditory bones of water animals, similar to water birds, are massive and shapeless, raptorial animals have slender bones [5]. Primates have long bones, i.e. the malleus and the incus are more slender and less massive than in humans (179). Monkeys' stapes are narrower than humans'. Bone size is connected with the size of the animal, however, it is not directly proportional to it and for example rats' malleus is 2.5 mm long, the incus – 1.5 mm long and the stapes – 1 mm long, which shows they constitute on average about 25 per cent of the length of the human bones [3].

The system of the auditory ossicles is responsible for sound strengthening. The acoustic wave passing environmental borders (aerial and aqueous) loses about 99% of its energy. Its strengthening starting on the transmission stage is therefore essential [6,8]. The pinna, focusing the acoustic wave inside the auditory canal, is responsible for sound strengthening, sound resonance in the external auditory canal, the mechanism of levers of the auditory ossicles and the quasi-hydraulic mechanism between the tympanic membrane and the basis of the stapes. The last two are the most effective. The malleus and the incus form a system of one-arm lever, the arm of strength apposition (the manubrium of the malleus put into motion by the motion of the tympanic membrane) and the arm of the weight apposition (short crus of the incus) are spread along the rotation axis, which is in the middle of the head of the malleus and the trunk of the incus. Whereas, the strength apposed to the tympanic membrane, strengthened by the lever mechanism mentioned above, is directed to a manifold smaller basis of the stapes, which gives additional benefits in strength.

The available data do not provide us with the information concerning the evaluation of effectiveness of these mechanisms in animals, as there are almost no anatomical investigations concerning the auditory and balance organ in animals. The few papers by Wysocki et al. [11] are not sufficient.

The aim of the article is to analyse anatomical bases for the effect of sound strengthening. It was decided to measure the lever arms made of by the handle of the malleus and the long limb of the incus, as well as the surface of the stapes basis and the tympanic membrane.

MATERIAL AND METHODS

Temporal bones of nine species of mammals were examined: humans, Macaques, dogs, cats, American Minks, cattle, rabbits, guinea pigs and rats. These species belong to the following orders: primates, carnivores, ungulates or hoofed animals, rodents and lagomorphs. 20 bones of each species (taken from 10 females and 10 males) were studied (5 left and 5 right ones). The temporal bones came from the collection of the Institute of Physiology and Pathology of Hearing. Having been fixed in 10% formaldehyde, the bones were prepared with the use of an operation microscope, dental drill and microsurgical set of tools. The measurements were conducted using MultiScan computer programme. The external auditory meatus was resected. The surface of the tympanic membrane was measured. The auditory ossicles were isolated and measurements were made of the length of the part of the handle of the malleus and the long limb of the incus which were situated distally to the rotation axis of the ossicles, going through the middle of the head of the malleus and the body of the incus. Finally, the surface of the basis of the stapes was measured. The measurements were made with the accuracy to 0.05 mm.

RESULTS

Average lengths of the arms of the lever of particular species of mammals are presented in Table 1, Table 2.

Multiplying both averages from Table 1 and Table 2, total mechanical strengthening of the sound wave was calculated. For humans it was – 23.56, for Macaque – 29.53, for cattle – 50.1, for dogs  68.09, for cats – 94.04, for American  Minks – 81.7, for rabbits – 44.34, for Guinea pigs – 33.65 and for rats – 20.77 (Fot. 1–9).

DISCUSIONS

There are no data available for comparison with results discussion in the present writing. It is observed that there is no basic research based on measurements of animal preparations.  It is only possible to refer to literature data concerning behavioral studies suggesting the scope of the auditory area. Thus, the auditory surface of humans is from 16 Hz to 20 kHz, dogs – 67 Hz – 45 kHz, cats – 45 Hz – 64 kHz, cattles – 23 Hz – 35 kHz, American Minks – 16 Hz – 48 kHz, rabbits – 360 Hz – 42 kHz, rats – 200 Hz – 76 kHz and guinea pigs – 54 Hz – 50 kHz [14,15,17,18]. The scope of frequency of audible sounds does not have any influence on sensitivity of the hearing but it increases  opportunities of perceiving sounds from the surroundings. Thus, the species of animals, about which we have some data collected, show significant advantage over humans.

Within the hearing intensity, the  sound scope of animals is from  -30 dB to 120 dB, therefore, it is wider than the one of humans' (0–100 dB), especially in carnivores.

It was discovered  that carnivores had the best conditions for sound strengthening through the conducting apparatus of the middle ear (lever mechanism multiplied by hydraulic mechanism – see the table). At good conditions for sound strengthening and at relatively broad scope of audible frequencies, it makes them the best as far as agility of the hearing organ was concerned.

CONCLUSION

Carnivores have the best anatomical conditions for sound strengthening in the middle ear of all the studied species.

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Accepted for print: 4.05.2011

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