AND PHYSIOLOGY 



53 



from soVoth to - T VTrd of an 

 inch in diameter. The en- 

 amel is also covered by a very 

 thin membrane, TI f 5 7 st of an 

 inch in diameter, called Nas- 

 myth's membrane. This is 

 a " calcified" membrane and 

 can be seen only with great 

 care, since it is not acted up- 

 on by the strongest acids or 

 alkalies. 



121. Development of Teeth. 

 ^Temporary Set. Permanent 

 Set. — The teeth in the human subject are not perfectly formed 

 at birth, but exist in the form of follicles or shut sacs, which 

 at the seventh or eighth month of infantile life, are developed 

 into teeth. The process of dentition generally occupies from 

 one to three years, during which time the temporary set, as 

 it is termed, make their appearance. These number twenty, 

 ten in each jaw, and between the age of seven and fourteen 

 become loose, and are easily removed to give place to the per- 

 manent set which numbers thirty-two. Generally, however, 

 the last tooth in each jaw does not make its appearance until 

 the twentieth year of life. 



122. A more accurate statement of dentition of the tempo- 

 rary teeth shows that they appear at the following ages : 



The Incisors, from the fall to the 10th month. 

 The Anterior Molars, 12th " 13th " 

 The Canine Teeth, 14th " 20th " 

 Posterior Molars, 18th " 36th " 



123. The permanent teeth appear as follows : 



Fig. 65. 



A portion of the Surface of the Enamel 

 on which the Hexagonal Terminations of 

 the Fibres are shown ; highly magnified. 

 1 2, 3, Are more strongly marked dark, 

 crooked Crevices, running between the 

 rows of the Hexagonal Fibres. 



Describe each of the teeth. What membrane completely covers the outside of the 

 tooth? 121. Illustrate the development of the teeth. 122. Give the time of appearance 

 of the temporary teeth. 123. When do the permanent teeth appear ? 



