2 5 2 NATURAL SCIENCE. April, 



portion of it, is invested with cement ; and, shortly before cutting the 

 gum, the inner layer of enamel belonging to the infundibulum becomes 

 lined with a layer of cement, the thickness of which is subject to 

 considerable individual variation. The infundibulum is conical in 

 form, the apex being directed towards the root-end of the tooth ; and, 

 since the area of the section of the infundibulum is greatest towards 

 the biting end of the tooth, one may roughly conclude that the 

 smaller the size of the mark the greater is the age of the tooth. 



The central permanent incisors cut the gum before the second or 

 intermediate, and the second before the third or corner incisors ; and, 

 since these teeth wear away at approximately the same rate, the 

 mark disappears in the same order, first on the centrals and last on 

 the corners. The mark is lost in the central lower incisors between 

 five and six years, in the intermediates between six and seven, and in 

 the corner teeth between seven and eight years ; but, since the dis- 

 appearance of the marks depends so much on the nature of the food 

 and on the amount of cement lining the infundibulum, the modern 

 tendency is to place less reliance than formerly on this character 

 when estimating the age of a horse. Owing to the greater depth of 

 the infundibula in the upper incisors than in the lower, the marks 

 are retained for a longer period ; they are obliterated between eight 

 and twelve years of age, but with less regularity than in the lower 

 jaw. After the blackened mark has disappeared, the table still 

 exhibits an area of cement surrounded by the central enamel. This 

 usually disappears in the lower incisors at twelve or thirteen years, 

 but the central enamel may sometimes still be seen at twenty or even 

 later. After the central enamel has disappeared, the table exhibits 

 only a ring of primary dentine, bounded externally by the peripheral 

 enamel, and surrounding a central area of secondary dentine of 

 darker and yellower tint. This is known as the dentinal star, and is 

 formed by the filling up of the pulp-cavity by secondary dentine. 

 The dentinal star begins to appear at eight years in the central 

 and intermediate lower incisors, in front of the ring of central 

 enamel. 



In a recently cut incisor the exposed crown is broader trans- 

 versely, but towards the middle and root end the section is longer in 

 the antero-posterior direction, so that the age of the horse may be 

 roughly determined by the position of the long diameter of the table. 

 In the lower central and intermediate teeth the tables are oval, with 

 the long diameter transverse, until fourteen years, when they become 

 triangular ; and at nineteen years the longer diameter of the table is 

 parallel to the length of the head. 



The lower corner tooth is very often a " shell tooth," the 

 posterior border being entirely absent, so that the infundibulum is 

 widely open behind. 



The incisors of the horse, like the cheek teeth, are remarkable 

 for their great length of crown, and as the teeth gradually emerge 



