62 ANIMAL DENTISTRY. 



objects, but become tooth-like very rapidly during the first 

 twelve months, at vvrhich time they are all morphologically 

 and anatomically similar. During the ensuing year their 

 growth is about equal, each growing to the level of the 

 alveolar border. At two and a half years the first and sec- 

 ond expel the temporary shell and become concerned ac- 

 tively in mastication at about three years of age. The third 

 at two and a half years seems to be mysteriously retarded 

 in its growth as if blocked in its outward course by the 

 unyielding fourth molar, which has now been in active serv- 

 ice for the past one and a half years. (See osteoma of the 

 maxilla.) At two to two and a half years old the develop- 

 ment of the third molar is at a standstill, tintil the age of 

 three to three and a half years, when it finally expels its shell 

 and then grows tardily outward to the level of the arcade. 

 It is not unusual to find this molar below the level of its 

 neighbors at four and even five years old. 



Fourth Molar — The fourth molar is the most interesting 

 of the horse's teeth. Its development begins during the last 

 months of foetal life. At birth it consists of a small, rounded 

 mass of rudimentary tooth substance, developing from a 

 backward projection of the common enamel germ, and lo- 

 cated in close contiguity to the fang of the third temporary 

 molar. During the first ten months after birth it grows 

 very rapidly into a shapely tooth and points to the surface 

 at about the end of the tenth month. At twelve months it 

 is already actively concerned in mastication, although it is 

 morphologically defective, assuming its maximum stability 

 only during the next two years. 



Fifth Molar — The common enamel germ at the age of 

 six months, after having formed the fourth molar projects 

 backward to form the fifth molar. At one year old the for- 

 mation is almost perfect, at one and a half years it erupts, 



