Posts Tagged ‘hearing’

Development of Sound Signalling

Monday, December 14th, 2009

When life began on our planet, profound silence pre­vailed. The only sounds to be heard were those of thunder­bolts and the roar of breakers against the sombre desolate dills by the primordial seas, but those sounds were of no interest to most animals. It was only after the animals themselves had achieved a higher stage of evolution and learned to wander about and devour one another that faint sounds began to be distinguished on the Earth. They were sounds of a biological origin produced by animals themselves. This prompted the development of the acoustic analyser and the sound signalling.

Extremely multifarious receptor devices, from wide-range to those only detecting a very narrow band of sounds, had to be evolved to cope with the gamut of sound sources.

Certain bats can best hear very high-pitched sounds of up to 300 kilocycles, but they can also hear very low sounds. Their auditory organ has a range of fifteen octaves. The nocturnal moths on which these bats feed have no use for such an enormous sound range. The tympanic organ of their wings is only able to pick up the ultrasonic signals of the bats. The organ for this limited function is simi­larly very simple in design. It consists of a membrane, air-sacs and two sensory nerve cells. Their sole function is to perceive the sound produced by the bat and to give the command so that the moth quickly changes direction of flight.

Man’s Principal Sense Organs

Monday, December 14th, 2009

Of the six principal sense organs three are the most important to us. We can lose the sense of taste and, cer­tainly, smell, and not even bother about it. One could even somehow put up with the loss of the sense of touch, but the loss of vision, hearing, or sense of equilibrium seriously incapacitates a person. These are the principal senses we use to perceive the world we live in.

Man’s main sense organs do not always coincide in importance with the main analysers in animals. Many repre­sentatives of the animal kingdom have very feeble vision and some are completely blind. Others are deaf, or only hear very badly, and yet they still manage quite well. As to the organ of equilibrium, this is an extremely important analyser and almost all multicellular animals have it.

Even in unicellular animals zoologists have discovered certain structures remotely resembling the organ of equilibrium of higher animals. The parasitic infusoria possess a device of this sort. They have a special vacuole, a small peripheral sac with some crystalline inclusions, similar in structure to the statocyst (the sac of the labyrinth maintaining static equilibrium) in multicellular animals. If it is some day discovered that the vacuole does indeed fulfill the same function, it will not be surprising. There are many remote corners on our planet plunged in pitch-darkness, many spots where no sound ever penetrates, but the Earth gravity is ubiquitous and inescapable.

There are grounds for believing that light has played an active part in the origination of life. At any rate, light sensitivity, which seems to have been possessed even by primary living matter, soon gave rise to a special organ, the organ of vision. Even contemporary unicellular flagel-lata perceive light. The unicellular animals, particularly the Peridinia, many of which can themselves emit light, have rather large eyes. Their eyes are a bowl-shaped accumu­lation of reddish, fat-like, light-sensitive pigment located in the anterior part of the Peridinia, at the base of the flagellum. In the pigment there is a transparent grain of starch which serves to refract and focus light.

Of the three main sense organs which are most essen­tial to man, the two oldest are the organs of vision and equilibrium. These organs which are, on the whole, very dissimilar, have one interesting feature in common. Although the organs of vision and equilibrium have greatly modi­fied in the process of their evolution and perfection, they differ less in design and specific function than the acoustic analysers and the sound receptors in various animals. This is obviously because the organs of vision and equilibrium were both shaped under the impact of some single, constant global factor; the sense of equilibrium was formed under the influence of the Earth gravity, and vision under the impact of sunlight. But on the Earth there has never been any unique and standard source of sound.