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Hair
is a natural growth which kept human beings warm when life was very
primitive. Hair is made of the protein keratin, as are claws, fingernails,
feathers, fur, wool, porcupine quills, and bird beaks. Keratin contains
the sulfur containing amino acid cystine and the disulfide bond provides
some strength and stiffness to keratin. This accounts for the sulfur
peak in the PIXE spectrum. Hair is composed of an outer layer called
a scale. This scale is filled with strands of keratin called cortical
cells. These can sometimes be seen in highly magnified electron microscope
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