Chap. XIV] THE DIGESTIVE SYSTEM 259 



is continuous with a canal that traverses the centre of each root, 

 and opens by a small aperture at its extremity. It is filled with 

 dental pulp, which consists of loose connective tissue holding a 

 number of blood-vessels and nerves which enter by means of the 

 canal from the root. 



There are two sets of teeth developed during life : the first, 

 temporary or deciduous ; and the second, permanent. 



Temporary teeth. — In the first set are twenty teeth, ten in each 

 jaw. The cutting of the temporary teeth begins usually at seven 

 months and ends at about the age of two and one-half years. ^ 



Permanent teeth. — In the second set are thirty-two permanent 

 teeth, sixteen in each jaw. During childhood the temporary 

 teeth are replaced by the permanent. The first molar usually 

 appears at six years of age. 



According to their shape and use the teeth are divided into 

 incisors, canines, premolars, and molars. 



Right Middle Left 



Molar Premolar Canine Incisor Canine Premolar Molar 

 Upper ... 3 2 1 4 1 2 3 



Lower ... 3 2 1 4 1 2 3 



(ColHns.) 



Incisors are eight in number and form the four front teeth 

 of each jaw. They have wide, sharp edges, and are specially 

 adapted for cutting food. 



Canines are four in number, two in each jaw. The upper 

 canines are commonly called eye-teeth, the lower, stomach teeth. 

 They have a sharp, pointed edge and are longer than the incisors. 

 In the human animal they serve the same purpose as the incisors. 



Premolars (or bicuspids) are eight in number in the permanent 

 set, but there are none in the temporary set. There are four in 

 each jaw, two being placed just behind each of the canine teeth. 



They are broad, with two points or cusps on each crown ; these 

 teeth have only one root, the root, however, being more or less 

 completely divided into two. Their function is to cut and grind 

 food. 



' The temporary teeth are usually cut in the following order, the teeth of each 

 group appearing first in the lower jaw : central incisors, 7th month ; lateral inci- 

 sors, 7th to 10th month ; front molars, 12th to 14th month ; canine, 14th to 20th 

 month ; back molars, 18th to 30th month. 



