344 



DIGESTIVE SYSTEM OF THE HORSE 



that of the second is ahnost vertical, while in the remainder it is curved backward 

 in an increasing degree. The average length at six years of age is about three to 

 three and a half inches (ca. 7 to 9 cm.). The buccal (or outer) surface presents a 

 central ridge running lengthwise, antl se]:)arating two grooves; the first tooth has, in 

 addition, a less prominent ridge in front of the primary one. The lingual (or 

 inner) surface is marked by a wide, rounded ridge, the accessory pillar or column, 

 which separates two very shallow grooves. The masticatory or table surface 

 presents two infundibula, an anterior and a posterior. It slopes obliquely down- 

 ward and outward, so that the outer edge is prominent and sharp. The first and 

 last teeth have three roots, the remainder four or three. 



The position of the embedded crowns and roots of the last four varies at 

 different ages and in different subjects. Two factors in this variation may be 

 noted. All these teeth are developed in the maxillary sinus close to the orbital 

 fossa. As growth proceeds the teeth move forward, so that commonly only the 

 last three, but sometimes also the third, remain (except as to their free crowns) 

 in the sinus. The second cause of variation is the fact that the anterior limit of 

 the maxillary sinus may be at the extremity of the facial ridge, or more than an 

 inch beyond it. In the latter case the third tooth projects into the sinus.^ 



A B 



Fig. 252. — Cross-sections of Cheek Teeth of Horse. 

 Buccal (outer) surfaces face to left. A, Upper tooth, B, lower tooth; /, anterior, /', posterior infundibulum, both 



almost filled up with cement. 



The structure is quite complex. Two infundibula run vertically through the 

 entire lengtli of the crown; these become filled with cement. There are five main 

 divisions of the pulp-cavity and five enamel folds, four of which are arranged 

 symmetrically, while the fifth is an outgrowth from the inner side of the antero- 

 internal fold. On the exposed crown of the unworn tooth the enamel folds form 

 rounded ridges covered with a thin layer of cement. After the tooth comes into 

 wear the enamel on the masticatory surface stands out in the form of sharp 

 prominent ridges. Progressive cementation of the periphery of the tooth takes 

 place, thus leveling up the irregularities of surface to a considerable extent. 



The lower or mandibular cheek teeth are implanted in the rami of the mandi- 

 ble, forming two straight rows which diverge behind. The space ])etween the rows 

 is considerably less than that separating the upper teeth, especially in the middle 

 of the series. The length of the lower teeth is about the same as that of the upper 

 set. Their direction is also similar, but the embedded portions diverge even more, 

 with the exception of the first and second. The long axis of the first is vertical; 

 the remainder project downward and backward in a gradually increasing obliquity. 

 The buccal (outer) surface has a longitudinal furrow; the last molar has a secon- 



1 The student is advised to amplify these very general statements by the examination of 

 heads of subjects of varying ages. It may also be noted that the position of the septum betweert 

 the two divisions of the sinus varies much. 



