400 DIGESTIVE SYSTEM OF THE HORSE 
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 em.) 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 
Dentine Cement crown is remarkably long, most of it beimg em- 
bedded in the bone or projecting into the max- 
Enamel 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 
Gener 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- 
Cement 
eh Enamel 
| 
+— Dentine 
Root canal 
Fic. 343.—Frontat Section oF UPPER Fic. 344.—Cross-section oF Upper CHEEK Tooru or Horse. 
Cueek ToorH or Horse. 3uceal (lateral) surface to left. J, Anterior, I’, posterior infundib- 
C, C, Pulp cavities. Infundibulum filled ulum, both almost filled up with cement. 
with cement. 
illa. 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 em.). 
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 
1 The question to which set these teeth belong is an open one. 
2 For teeth of this kind it is convenient to employ the terms functional crown and reserve 
crown. 
