The purpose of this study was to examine the phonatory characteristics of pig, sheep, and cow excised larynges and to find out which of these animal species is the best model for human phonation. Excised pig, sheep, and cow larynges were prepared and mounted over a tapered tube on the excised bench that supplied pressurized, heated, and humidified air in a manner similar to that for excised canine models. Each excised larynx was subjected to a series of pressure-flow experiments with adduction as major control parameter. The subglottal pressure, electroglottograph (EGG), mean flow rate, audio signal, and sound pressure level were recorded during each experiment. EGG signal was used to extract the fundamental frequency. It was found that pressure-frequency relations were nonlinear for these species with large rate of frequency changes for the pig. The average oscillation frequencies for these species were 220+/-57 Hz for the pig, 102+/-33 Hz for the sheep, and 73+/-10 Hz for the cow. The average phonation threshold pressure for the pig was 7.4+/-2.0 cm H(2)O, 6.9+/-2.9 cm H(2)O for the sheep, and 4.4+/-2.3 cm H(2)O for the cow.
Elastic characteristics of the pig, sheep and cow vocal folds were investigated through a series of in vitro experiments. Sample strips of the vocal fold tissue were dissected from pig, sheep and cow vocal folds and mounted inside a saline-filled ergometer chamber that was maintained at 37°C ± 1°C. Sinusoidal elongation was applied on the samples to obtain the passive force measurements. Force and elongation data from the samples were recorded electronically with a dual-servo system (ergometer). Stress-Strain data were compared to characterize the interspecies differences in the elastic properties of vocal folds. Pig vocal folds exhibited the most nonlinear stress-strain relationship, indicating the presence of a high level of collagen fibers. Cow vocal folds had the highest Young's modulus, but the tissue displayed a nearly linear stress-strain profile. Previous studies of phonation in these three species have indicated that pig larynges have the highest range of phonation frequencies, making them a good candidate for animal studies. The current study provides quantitative data for the elastic properties of the oscillating laryngeal tissue in these species and indicates that nonlinear behavior of these tissues may lead to wider oscillation ranges.
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