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i8 PHYSIOLOGY AND ANATOMY. 
There are several bones which form. the face. The princi- 
pal ones are the jaw bones, and those around the organs of 
seeing, smelling, and tasting. 
‘The teeth are nneen of Sesie. covered with a very hard 
substance called enamel. When this decays, or is: injured, 
the tooth is useless for strength. The first teeth gradually 
fall out within five or six years, and then the other set en- 
langeuaid Sbpcees, Salah us sheesh life if we only take 
eare of them. 
As we leave the’ teeth, the next principal, bone which we 
see is the back-bone, or spine. It is composed of twenty 
four pieces of bone; each of these pieces are called a verte- 
bre. These are joined by a soft elastic—which means, when 
any thing is bent or stretched, and the force which was used. 
was removed, returns to its first shape—substance called car- 
tilage, which enables us to bend our backs. These vertebrae 
are hollow, and contain the spinal nerve or marrow, and 
