The meaning and origin of the expression: Foam at the mouth

Foam at the mouth

What's the origin of the phrase 'Foam at the mouth'?

Dogs and other animals, including humans, affected by rabies foam at the mouth.

There are examples of forms of this phrase in Old and Middle English that date back to at least the first millennium. The Lindisfarne Gospels, 950 AD, have a reference to 'Spumat faeme'. The earliest version in a form that we can now readily understand is in Shakespeare's Julius Caesar, 1601:

" He [Caesar] fell down in the market-place, and foamed at mouth, and was speechless."