Never Too Small A Chest

In "The Real Jack Russell Terrier," Eddie Chapman writes:

"Don't think that a deep chested dog can get in just because he is narrow. Don't be taken in by the saying that 'a dog needs a good sized chest so that he has plenty of heart and lung room.' It is impossible to breed a working terrier with too small a chest. They will run as far and as long as any deep chested terrier, and will never suffer from a shortage of breath underground. In fact, quite the contrary, as they will have the advantage of having more room to manoeuver because of the smaller chest, so will be less restricted."