80 ANIMAL PHYSIOLOGY. [LESSON 26. 



substance without destroying the spots, by which we see that they are 

 nothing but pigment granules, deposited in the plates of the hair ; 

 they are frequently found in dark hairs, but vary very much in size 

 and form. Other dark spots which, at first sight, appear to be pig- 

 Fro. 120. 



FIG. 121. 



Plates from the cortical substance of the shaft of 



Human hair. Scales from the root of the hair. 



ment granules, prove to be, on closer examination, little cavities filled 

 with air. 



431. The root of the hair, especially the lower half of it, differs so 

 far from the foregoing description, which applies to the shaft, that 

 the scales, in this situation, appear as well formed elongated cells, 

 each one possessing a nucleus. In Fig. 120, scales from the shaft, 

 and in Fig. 121, from the lower half of the root, are shown ; those 

 from the shaft (Fig. 120) are flat and fusiform (spindled-shaped) 

 in shape; those from the root (Fig. 121) are broader, and generally 

 better formed, possessing a cylindrical, straight, or serpentine nu- 

 cleus. 



432. The medulla forms but a small portion of a hair ; it is the 

 central streak or axis, and extends from the bulb to the point, nearly. 

 It is cylindrical in shape, and composed of a congeries of nucleated 

 cells. If white hairs be boiled in caustic soda till they swell and 

 coil up, simple pressure will generally demonstrate the cellular struc- 



PIG 122 ture of the medullary cylinder, which 



is then sufficiently transparent for 

 transmitted light. The medulla is re- 

 markable for the great quantity of air 

 which it contains; by its means the 

 medullary cells are broken up into a 

 series of distinct cylinders, of all 

 lengths some being much shorter than 

 others. In examining a hair from bulb 

 to point, it is curious to observe the 

 many breaks which the medulla sus- 

 tains, owing to the presence of air. A 



White Hair of the head. representation is given (Fig. 122) of a 



white hair from the head, which has been treated with caustic soda 



