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I found the answer to my own question. Is it a voluntary action: Here’s what I got:
“But the biggest mystery of the body, cat purring, is still yet to be answered to everyone satisfaction. After all these centuries of living with cats, we still do not fully understand how or why the cat purrs. The only element about the purr known for sure is that it is a voluntary action – the cat can only purr when he or she wants to. It is thought that the purr is made by vibrating certain chords in the voice box, but just how is not known. It is thought the purr might be a mantra or a soothing charm for a cat, to calm not only the cat and put him or her in a good mood, but everyone around the cat, too. One exciting theory is the purr might stimulate the cat’s own natural bodily healing processes.
Cats (big and small) are the only animals who purr. Contrary to popular belief, purring isn’t speaking. The purr comes from two membrane folds, called false vocal cords that are situated in the larynx behind the actual vocal cords. Cats purr 26 cycles per second, the same as an idling diesel engine. Cats purr both when inhaling and exhaling, keeping the mouth completely closed. Scientists believe purring is produced by blood in a large vein in the chest cavity that vibrates and is then magnified by air in the windpipe. Kittens are born blind and deaf, but the vibrations of their mother’s purring is a physical signal that the kittens can feel. It acts as a homing device, signaling them to nurse. Kittens begin purring at about one week old; then it’s a signal to the mother cat that they’re getting their milk and are content. Since purring is non-vocal, it doesn’t interfere with the suckling. Contrary to another popular belief, cats don’t purr purely for pleasure. Be alert: a deep purr can indicate that a cat is in pain or distress. Female cats will purr when in labor. Cats may purr in fear or anxiety. They will also purr in anticipation of being fed or stroked. Big African cats only purr in short bursts, but the house cat can purr for hours.
http://www.sniksnak.com/cathealth/whydo….
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Years ago, it was taught that it was their blood moving around that did it.. but I found this online that is more current:
No one knows for sure why a domestic cat purrs, but many people interpret the sound as one of contentment. Our understanding of how a domestic cat purrs is becoming more complete; most scientists agree that the larynx (voice box), laryngeal muscles, and a neural oscillator are involved.
Kittens learn how to purr when they are a couple of days old. Veterinarians suggest that this purring tells ‘Mom’ that “I am okay” and that “I am here.” It also indicates a bonding mechanism between kitten and mother.
As the kitten grows into adulthood, purring continues. Many suggest a cat purrs from contentment and pleasure. But a cat also purrs when it is injured and in pain. Dr. Elizabeth Von Muggenthaler has suggested that the purr, with its low frequency vibrations, is a “natural healing mechanism.” Purring may be linked to the strengthening and repairing of bones, relief of pain, and wound healing (See Web link to Felid purr: a healing mechanism).
Purring is a unique vocal feature in the domestic cat. However, other species in the Felidae family also purr: Bobcat, Cheetah, Eurasian Lynx, Puma, and Wild Cat (Complete list in Peters, 2002). Although some big cats like lions exhibit a purr-like sound, studies show that the Patherinae subfamily: Lion, Leopard, Jaguar, Tiger, Snow Leopard, and Clouded Leopard do not exhibit true purring (Peters, 2002).”
What makes the purr distinctive from other cat vocalizations is that it is produced during the entire respiratory cycle (inhaling and exhaling). Other vocalizations such as the “meow” are limited to the expiration of the breath.
It was once thought that the purr was produced from blood surging through the inferior vena cava, but as research continues it seems that the intrinsic (internal) laryngeal muscles are the likely source for the purr. Moreover, there is an absence of purring in a cat with laryngeal paralysis. The laryngeal muscles are responsible for the opening and closing of the glottis (space between the vocal chords), which results in a separation of the vocal chords, and thus the purr sound. Studies have shown, that the movement of the laryngeal muscles is signaled from a unique “neural oscillator” (Frazer-Sisson, Rice, and Peters, 1991 & Remmers and Gautier, 1972) in the cat’s brain.
http://www.loc.gov/rr/scitech/mysteries/…