What is purring?

According to research posted by the American Association for the Advancement of Science, the purr is something cats are able to do from birth when they purr primarily while suckling. Purring is used in a wide variety of circumstances, not just when a cat is happy. For instance, veterinarians have noticed that some cats purr continuously when they are chronically ill or appear to be in severe pain. It is thought that they do so as a way to solicit care from humans.   

Others are of the opinion that a cat purrs when ill or in pain to ward off threats.  If a cat is ill in the wild, he may purr when approached by another cat, so the approaching cat doesn't feel he is a threat and attack.

Cat purrs range from a deep rumble to a raspy, broken sound, to a high-pitched trill, depending on the cat's  mood and/or physiology.  Many cats will "wind-down" when going to sleep, with a long purring sigh that drops melodically from a high to a low pitch.  A cat purrs at roughly the same velocity of idling diesel engine - around 26 cycles per second.
How does a cat purr?

There has been a lot of speculation on how purring occurs.  According to some, a purr is created by the vibration of a cat's vocal cords when it inhales and exhales.  Others feel it is caused by soft palate vibrations.  Some have wondered if cats have a set of false vocal cords within the larynx. 

Some researchers theorize it is a vibration caused by blood passing through the large veins in the cat's chest cavity, amplified by the diaphragm, which passes up the windpipe and into the windpipe and into the sinus cavities of the skull. 

Electromyographic tests - they measure the level of electrical activity in muscles - seem to indicate it is caused by the activation of the muscles of the larnyx, and partial closure of the glottis (the opening of the larnyx). 

The most recent declarative statement found on the web was made by Katharine Houpt, director of the Animal Behavior Clinic at Cornell University.  In 2002, she is quoted as saying:  "It's a vibration of the larynx that resonates down to the windpipe and into the diaphragm.  Unlike meowing or human speech, purring isn't the result of air passing over the vocal cords."

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