326 



ANATOMY OF VERTEBRATES. 



than in Man, especially the middle pair; but the dispropor- 

 tionate size of these is still more manifest and characteristic of 

 25 8 the Orang. The crown 



of the canine is longer 

 and more pointed in the 

 Chimpanzee-than in Man ; 

 still more so, and further 

 apart from the incisor in 

 the Orang. The first 

 milk-molar, fig. 258, d 3, 

 in the human subject is 

 more similar in shape and 

 size to the second, d 4, 

 than it is in either the 

 Chimpanzee or Gorilla: 

 in which it is relatively 

 smaller, showing in the 

 lower jaw a subcom- 

 pressed triangular crown. 

 The eruption of the 

 human milk-teeth usually 

 begins in the infant of seven months old, and is completed about 

 the end of the second year; those of the lower jaw preceding the 



Deciduous and permanent teeth, Human Child : ajt. cJ. 



Highly-magnified section of dentine and cement, from the fang of a Human molar, v, pi. 123. 



upper. The average periods of the appearance of both decidu- 

 ous and permanent teeth are as follows : — 



Permanent teeth. 

 6i years, first molar, m 1, (fig. 258). 



Deciduous teeth. 

 7th month, mid-incisor, d i 1. 

 ib. to 10th month, lat.-incisor, d i 2. 

 12th to 14th month, first molar, d 3. 

 14th to 20th month, canine, d c. 

 18th to 36th month, second molar, d 4. 



7th year, mid-incisor, i 1. 



8th year, lat.-incisor, i 2. 



9th year, first bicuspid, p 3. 



10th year, second bicuspid,^? 4. 



11th to 12th year, canine, c. 



12th to 13th year, second molar, m 2. 



17th to 21st year, third molar, m 3. 



The structure of human dentine is exemplified in fig. 259. 



