256 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



seventeen years' friction. The shape of the wearing surface of the tooth 

 is of great importance in determining approxi- 

 mately the age of the horse. Before eight 

 years the eruptive changes and periodic appear- 

 ances of the teeth are very regular and valuable in 

 indicating age. The foal at birth indicates the 

 fast approaching eruption of the two central 

 incisors. Sometimes these are through the gum 

 when the animal is foaled ; if not they appear 

 within the first month. Three molar teeth on 

 each side of both upper and lower jaws are prom- 

 inent, and in apposition for wear at the same time. 

 One incisor on each side of the two central ap- 

 pears at six weeks, and then time is allowed for 

 the jaw to grow. The cavities of reserve with 

 teeth forming in them grow behind the teeth first 

 formed, and by nine months the corner incisors 

 appear, and gradually grow until the animal is a 

 year old, when all the colt's incisors are in full 

 use. Within one and two years of- age little can 

 be seen beyond a gradual wearing down of the 

 temporary teeth, and the protrusions through the 

 gums of the fourth molar on each side of the two 

 jaws. At two years the worn aspect of the 

 incisors indicates the approaching displacement 

 of the central ones, and the fifth molar protrudes 

 through the gums. 



Between two and three years the central per- 

 manent incisors displace the temporary, and are 

 readily recognized b3^ their size, yellowish color 

 of enamel, and dark infimdibulum. At this age 

 the middle incisors are often knocked out to make 

 the horse look "three off" or "coming four.'' 

 This often retards their eruption, which is always 

 complete at four years, when the sixth molar tooth 

 on either side of both jaws is also advanced 

 through the gum. By this time the three tempo- 

 rary molars, or grinders, which are noticed shortly 

 after birth, have given way to permanent teeth. 

 The lower tushes are felt through the membran* 

 between the corner and first molar as early as 

 three years of age, but they only appear above it 

 It is at this age that the horse's mouth becomes 



Fig. 101.— Teeth of 

 Horse. (Gamgee) 



A. Longitudinal section of per- 

 manent incisor of horse shortly 

 after eruption. 



B. Transverse sections of per- 

 manent incisor of horse, showing 

 the way the table becomes triangu- 

 lar from wear, a, on eruption ; o, 

 at two years ; r, at five years : d, at 

 nine years ; c, at ahon't seventeen 

 years of age. 



C. Table of molar tooth of horse, 

 showing longitudinal arrangement 

 of cement in grooves. 



between four and five. 



