99: ELEMENTS OF APPLIED MICROSCOPY. 
of known composition furnishes the only sure criterion 
for identification. 
10. Wool and Other Animal Hairs.—Animal hairs are 
much more complex structures than those of plants, being 
not only multicellular, but composed of several distinct 
layers of cells. In the growth of a hair the epidermis or 
outer tissue of the skin is first folded in to form a minute 
pit called the hair-follicle. From the base of this follicle 
the hair grows out, its cells dividing off from the epider- 
mal tissue below. The hair itself is made up of three 
distinct zones. In the center is an axis of irregularly 
rounded cells known as the medullary layer. Outside 
this, and forming the main portion of the hair, is the cor- 
tex, which consists of elongated spindle-shaped cells; 
and at the periphery is a cuticular layer of flat overlapping 
scales covering the hair like the shingles on a roof. In 
the cortical cells is situated the pigment which gives to 
the hair its color, and to the disintegration of these cells 
and the formation of air-spaces in their place is due the 
whitening of the hair in old age. 
Externally the principal characteristic of the animal 
hair is the presence of the scales of the cuticle which give 
the fibre the effect of being marked with fine transverse, 
anastomosing lines and which at its edge (Fig. 37) appear 
as fine serrations. The medullary cells may just be 
made out beneath this cuticle as fine longitudinal stria- 
tions. 
The general term wool is applied to hairs which are 
spirally twisted so that when woven they will hold to- 
gether to form a strong fabric. In such hairs, too, the, 
