occurs on the upper corner incisor, producing a vertical line, and is helpful in approximating the age of older horses. It generally first appears at age 10, reaches halfway down the tooth by age 15, and is completely down the tooth at age 20. It then begins to disappear, usually half-way gone by age 25, and completely gone by age 30. See Sydney Frederick Galvayne. Angle and Shape of the incisors: As the horse ages, the angle of the incisors generally becomes more acute, slanting forward. The incisors gradually change their form as the horse ages, becoming round, oval, and then triangular. Continuous eruption and loss A horse's incisors, premolars, and molars, once fully developed, continue to erupt as the grinding surface is worn down through chewing. A young adult horse's teeth are typically 4.5–5 inches long, but the majority of the crown remaining below the gumline in the dental socket. The rest of the tooth slowly emerges from the jaw, erupting about 1/8" each year, as the horse ages. When the animal reaches old age, the crowns of the teeth are very short and the teeth are often lost altogether. Very old horses, if lacking molars to chew, may need soft feeds to maintain adequate levels of nutrition. Older horses may appear to have a lean, shallow lower jaw, as the roots of the teeth have begun to disappear. Younger horses may seem to have a lumpy jaw, due to the presence of permanent teeth within the jaw. The teeth and the bit If a bit is fitted to a horse, along with a bridle, the normally metal bar of the bit lies in the interdental space between the incisors (or canines, where present) and premolars. If the bridle is adjusted so that the bit rests too low, or too high, it may push against the teeth and cause discomfort. Sometimes, a "bit seat" is filed in the first premolar, where the surface is rounded so that the flesh of the cheek is not pushed into the sharp edge of the tooth, making riding more comfortable for the horse, although the practice is controversial. Dental problems Like all mammals, horses can develop a variety of dental problems, with a variety of dental services available to minimise problems through reactive or prophylactic intervention. Equine dentistry can be undertaken by a vet or by a trained specialist such as an equine