Normal?

Question
My Neighbor has just been given a ten year old Jack Russel by an old lady. I have looked at him and he seems healthy. I am worried about him because my neighbor suffers from depression and sometimes losses it. She used to have a  rottweiler. She penned him in and he became aggressive. One day the dog was gone. She never told us what happened to him. The main thing im concerned about is that he shakes all the time. She told me that his done it all his life, and the vet says his ok. Is this normal? Is he scared or ill?

Answer
Ten year old dogs can shake for a number of reasons, pain, fear, illness, joint issues.His teeth could hurt if they have never been cleaned. His heart and lungs could hurt if he has never been on heartworm prevention. He might be scared after losing his home of ten years.

Unless you have a vet look him over there is no way to know what it is.
Dogs do not shake all their lives unless they had distemper as a puppy and recovered. Then they CNS or central nervous system issues for life that can lead to tremors.

But as I said, there is no way to know why he shakes without a checkup.