400 



DIGESTIVE SYSTEM OF THE HORSE 



Dtnlinc 



Cement 



twenty-four — twelve in each jaw. Quite commonly, however, the number is in- 

 creased by the presence in the upper jaw of the so-called "wolf-tooth." This 

 tooth, the first premolar, is usually situated just in front of the first well-developed 

 tooth; it is a much-reduced vestige, not often more than one-half or three-fourths 

 of an inch (ca. 1 to 2 cm.) in length. (It is interesting as being the remnant of a 

 tooth which was well developed in the Eocene ancestors of the horse.) It may 

 erupt during the first six months, and is often shed about the same time as the 

 milk-tooth behind it, but may remain indefinitely. The occurrence of a similar 

 tooth in the lower jaw — which rarely erupts — increases the dental formula to 44, 

 which is considered the typical number for mammals.^ The cheek teeth are very 

 large, prismatic in form, and quadrilateral in cross-section, except the first and 



last of the series, which are three-sided. The 

 crowTi is remarkaljly long, most of it being em- 

 bedded in the bone or projecting into the max- 

 illary sinus in the young horse. As the exposed 

 part wears away the embedded part erupts to 

 replace it, so that a functional crown of about 

 four-fifths of an inch (ca. 2 cm.) is maintained.^ 

 The root begins to grow at about five years 

 of age, and is complete at twelve to fourteen, 

 although the deposition of cement may con- 

 tinue indefinitely. 



The maxillary or upper cheek teeth are 

 embedded in the alveolar processes of the max- 



Ciment 



Enamel 



DclditiC 



Enamel 



Cement 



Roof canal 



Fig. 34.3. — Froxtal Section of Upper 



Cheek Tooth of Horse. 



C. C, Pulp cavities. Infundibulum filled 



with cement. 



Fig. 344. — Cross-section of Upper Cheek Tooth of Horse. 

 Buccal (lateral) surface to left. J, Anterior, / ', posterior infundib- 

 ulum, both almost filled up with cement. 



ilia. The exposed parts of the crowns are normally in close contact, forming a 

 continuous row which is slightly curved, with the convexity toward the cheek. 

 The embedded parts diverge in the manner shown in the annexed figures (Figs. 342, 

 347). Thus the long axis of the first is directed upward and a little forward, that 

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

 an increasing degree; the last tooth in particular is strongly curved in the adult. 

 The average length at six years of age is a little more than three inches (ca. 8 cm.). 

 The buccal surface presents a central ridge running lengthwise, and separating two 

 grooves; the first tooth has, in addition, a less prominent ridge in front of the 

 primary one. The lingual surface is marked by a wide, rounded ridge, the acces- 

 sory pillar or column, which separates two very shallow grooves. The masticatory 



' The ciuestion to which set these teeth belong is an open one. 



- For teeth of this kind it is convenient to employ the terms functional crown and reserve 



