9° 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.. 



