Dentistry is the practical application of knowledge of dental science (the science of placement, arrangement, function of teeth and their supporting bones and soft tissues) to human beings. A dentist is a doctor qualified to treat diseases, malformations of, and injuries to teeth, a professional practitioner of dentistry. In most countries, several years of training in a university (usually 4-8) and some practical experience working with actual patients' dentition are required to become a qualified dentist. The patron saint of dentists is Saint Apollonia, martyred in Alexandria by having all her teeth violently extracted, not, one would have thought, such a very desirable exemplar.