NUTRITION PROPER. 297 



180, 1, 2, 3, etc.) show the embryonic development of 

 teeth in man. The same process is true of all mammals. 

 At first there is a little papilla on the gum, which is 

 the pulp of the future tooth (1). Then this is sunk more 

 and more into the gum, to become the future socket (2). 

 Then the pulp begins to secrete the tooth, and the socket 

 closes up above, forming the tooth (sac 3, 4, and 5), and 

 the tooth is now entirely inclosed in the gum and in the 

 jawbone. Then by continued growth it breaks through 

 the jaw and the gum and the tooth is cut, and thence 

 continues to grow to its full size (6 and 7). But some 

 teeth — milk teeth — are shed and replaced by permanent. 

 Are these also gum structures ? Yes. It is seen that 

 the tooth sac has a saccule on each side. One of these 

 continues to develop, and a tooth is formed in it. This 

 grows, and finally pushes out the first tooth and takes 

 its place (5, 6, 7). In some rare cases the other saccule 

 also forms a tooth, which may develop. It is in this 

 way that we account for those rare cases of a third set 

 of teeth. But what is exceptional in man is the rule 

 in many reptiles and in sharks, as we shall see here- 

 after. 



2. Composition of Teeth. — Mammalian teeth consist usu- 

 ally of three kinds of substance — viz., dentine, enamel, and 

 cement. The dentine is a denser kind of bone, already 

 described, and forms the principal part ; the enamel is a 

 still denser variety, and covers the crown or exposed 

 part ; and the whole is covered and the inequalities 

 filled up with cement, which is a more structureless 

 variety than either. The cement is almost wanting in 

 many teeth, as man's, and in such cases is quickly worn 

 off and disappears, but is an important part of the more 

 specialized teeth of many mammals. 



3. Kinds of Teeth. — There are four kinds of teeth in 

 the jaws of mammals — viz., (1) the incisors, or front 



