When a horse has all his permanent teeth there are six incisors on the top jaw and six on the bottom. These are known as two central incisors, two laterals and two corner incisors. There are six molars on each side of each jaw. In addition the male horse has four tushes, one in each gap between incisors and molars. This portion of the jaw is known as the bars and here the bit should rest, clear of the tushes.
When born, a foal has two central incisors just through the gum. The lateral incisors appear at about five weeks and the corner incisors at about eight months, when he has a complete set of incisors. The front three molars are there at birth or within a fortnight. All these teeth are milk or temporary teeth and are replaced as time goes on by permanent teeth. When this replacement takes place his age can be accurately assessed.
The two permanent centrals come through the gums at 2 and a half years and are in wear at three years.
The laterals are up at 3 and a half years and are in wear at 4 years. The corners are up at 4 and a half years and are in wear at 5 years.
All these new teeth have distinct cavities on their tables (the biting surface) and from these cavities the ageing is assessed for the next three years. These cavities wear flat in the centrals at 6 years, in the laterals at 7 years and from the corners at 8 years. The horse is then technically said to be 'aged'.
At 8 to 10 years a black Line, Galvayne's Grove, appears at the top of the corner incisor on the top jaw. This extends downwards each year, is about halfway down at 15 years and reaches the bottom at 20 years.
From 10 years onwards the incisors tend to protrude outwards and gradually become longer and narrower, - the young tooth is almost wider than it is long, it meets the gum squarely at the top, whereas the older tooth runs up into more and more of a point here.
The tables of the young tooth are oval in shape but from 10 years on this shape becomes more triangular.