ONE OF THE BEAUTY FISH
BLOWHOLE
Whales and dolphins must come to the surface to breath. In the course of evolution, the nostrils of the whales moved to the top of their head to make breathing at the water surface easier. In toothed whales, the air passages and the nostrils fused into a single air canal and blowhole. The blowhole is closed by a strong valve, which opens briefly for a fast exhalation and inhalation. Below the blowhole there is a system of air sacs, which help in closing the blowhole and which also play a role in sound production.Bottlenose dolphins can see equally well above and under water. The dolphin eye is optimised for underwater vision. In the human eye, most of the refraction is done by the cornea, while additional focussing is done by the lens. Underwater the human eye loses most of its refractive power and the lens cannot compensate for that. Underwater we cannot get a clear picture: we are extremely far-sighted underwater. In the dolphin eye, the refractive power of the lens has become greatly increased, because the lens is located further forward and is completely spherical. The dolphin eye looks a lot like a fish eye. 
The dolphin pupil is rather special: instead of a round hole that narrows in bright light, there is a kind of "lid" (called operculum) that slides down, covering the centre of the pupil, leaving narrow slits on its edges in bright light conditions. These narrow slits may give the dolphin more depth of vision above water and therefor better vision. Special adaptations in the edges of the lens may also improve above water vision.
The dolphin retina is organised differently than most mammal eyes: instead on one high-sensitivity area (or yellow spot), the dolphin eye has two such areas. One may be associated with forward vision and the other with lateral (sideways) vision.
D
olphins have tiny external ear openings, which are barely visible, just behind the eyes. These openings probably have no or only a limited function in hearing. The acoustic faculty in dolphins is well developed. The auditory systems in the brain are highly developed and much larger than for instance in humans. This strong development of the auditory systems of the brain is at least in part an explanation for the large brains in dolphins. The auditory nerve has double the amount of nerve fibres compared to the human auditory nerve. Bottlenose dolphins can hear sounds with frequencies between 75 Hz and 150 kHz (in humans the range is 10 Hz to 16-20 kHz). Dolphins are most sensitive for sounds between 40 and 70 kHz. In dolphins, the sound is conducted to the middle ear mainly via the blubber, which is an excellent sound conductor, and the lower jaw. The lower jaw of the dolphin is filled with a fatty tissue, which conducts sound quite well. This tissue extends from a thin area of the lower jaw to the inner ears. Experiments in which a sound-absorbing hood of neoprene was placed over the lower jaw showed that dolphins with the hood in place had considerable difficulty hearing. The middle ear cavities of dolphins are independently suspended and surrounded by air-filled spaces. This reduces the contact with the surrounding bone and can probably help the dolphin in directional hearing. The middle ear in dolphins serves 2 functions: one is to stiffen the sound transmission system, optimising it for high frequencies. The other is to balance the pressure between the inner ear and the external environment. The pressure of a given sound in water is about 60 times as high as the same sound intensity in air.
olphins have tiny external ear openings, which are barely visible, just behind the eyes. These openings probably have no or only a limited function in hearing. The acoustic faculty in dolphins is well developed. The auditory systems in the brain are highly developed and much larger than for instance in humans. This strong development of the auditory systems of the brain is at least in part an explanation for the large brains in dolphins. The auditory nerve has double the amount of nerve fibres compared to the human auditory nerve. Bottlenose dolphins can hear sounds with frequencies between 75 Hz and 150 kHz (in humans the range is 10 Hz to 16-20 kHz). Dolphins are most sensitive for sounds between 40 and 70 kHz. In dolphins, the sound is conducted to the middle ear mainly via the blubber, which is an excellent sound conductor, and the lower jaw. The lower jaw of the dolphin is filled with a fatty tissue, which conducts sound quite well. This tissue extends from a thin area of the lower jaw to the inner ears. Experiments in which a sound-absorbing hood of neoprene was placed over the lower jaw showed that dolphins with the hood in place had considerable difficulty hearing. The middle ear cavities of dolphins are independently suspended and surrounded by air-filled spaces. This reduces the contact with the surrounding bone and can probably help the dolphin in directional hearing. The middle ear in dolphins serves 2 functions: one is to stiffen the sound transmission system, optimising it for high frequencies. The other is to balance the pressure between the inner ear and the external environment. The pressure of a given sound in water is about 60 times as high as the same sound intensity in air. 
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