THE DECIDUOUS TEETH ERUPTION OF THE TEETH 403 



closed by deposit of cement. The pulp-cavity has two main divisions (anterior 

 and posterior) and three or four secondary diverticula. The enamel folds corres- 

 pond, forming a pattern even more complicated than on the upper teeth. 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 lengths of the lower teeth shown in Fig. 342 were in centimeters : 5.7, 7.9; 9,0, 8. .5, 8.2 

 7.6. The distance between the two rows was 4.7 cm. at the anterior end and 7 cm. at the posterior 

 end. 



The Deciduous Teeth 

 The deciduous teeth are smaller and fewer than those of the permanent set. 

 The formula is : 



/ 3 3\ 

 2 ( Di - Dc - Dp - ) = 24 

 V 3 3/ 



The deciduous incisors are much smaller than the permanent ones. They 

 have a distinct neck at the junction of the crown and root. The crown is short, 

 white in color, and its labial surface is smooth. The inf undibulum > is shallow. 

 The root is flattened; it undergoes absorption as the permanent tooth develops 

 behind it. 



The deciduous canines are quite vestigial. They occur in both sexes as slender 

 spiculae about a quarter of an inch in length, but do not erupt. The lower one 

 develops close to the corner incisor. They are not usually included in the formula, 

 as they are never functional. 



The deciduous premolars differ from the permanent set chiefly in that they 

 have much shorter crowns than the latter. The roots form early, so that a distinct 

 neck occurs. 



Vessels and Nerves. — The blood supply of the teeth is derived from the 

 infraorbital and alveolar branches of the internal maxillary artery. The lyrhph 

 vessels go to the mandibular and pharyngeal lymph glands. The nerves come 

 from the trigeminus. 



Eruption of the Teeth 

 The subjoined table indicates the average periods of the eruption of the teeth: 



Teeth Eruption 



A. Deciduous: 



1st incisor (Di 1) Birth or first week. 



2nd " (Di 2) 4r-6 weeks. 



3rd " (Di 3) 6-9 months. 



Canine (Dc ) 



1st premolar ^5^0^ Birth or first 



2nd Dp 3 ^^^gtg_ 



3rd " ; (Dp 4) J 



B. Permanent: 



1st incisor (I 1) "^H years. 



2nd " (I 2) S^A years. 



3rd " (I 3) 4^2 years. 



Canine : (C ) 4r-.5 years 



1st premolar (or wolf-tooth) (P - 1) 5-6 months. 



2nd " (P 2) 2J2 years. 



3rd " (P 3) 3 years. 



4th " (P 4) 4 years. 



1st molar (M 1) 10-12 months. 



2nd " (M 2) 2 years. 



3rd " (M 3) 3K-4 years. 



(The periods given for P 3 and 4 refer to the upper teeth; the lower ones may erupt 

 about six months earlier.) 



' It is estimated that the enamel ridges of an upper cheek tooth of a young adult horse, if 



straightened out, would form a Une more than a foot long. The extent diminishes with age. 



