74 ANIMAL PHYSIOLOGY. 



cavities, and in others, sncli as the pig, the medulla is 

 entirely absent. 



48. The shaft of the hair is in some instances cylindrical, and 

 in others flattened, and the tendency to curling is connected 

 with the form. Thus the straight hair of the North American 

 Indian is cylindrical, and the negro's woolly hair is quite flat. 

 The pigment, on which the colour depends, is diffused in 

 variable degree through cortex and medulla ; but the most 

 curious point with regard to the medulla is its connection 

 with the turning of the hair white. In that change there is 

 a disappearance of pigment; but there is likewise the develop- 

 ment in the medulla of numbers of closely set minute globules 

 of air. Such air globules are also constantly present in the 

 white hairs of other animals, and reflect the light from their 

 surfaces. A hair may begin to turn white in any part of its 

 course, or in patches at different points ; and sometimes one 

 may see in one hair some parts unchanged, some with 

 the pigment gone, but without air globules, others with 

 air globules, but with the pigment remaining, and parts with 

 both changes complete. It will be observed, therefore, that 

 the hair undergoes changes of nutrition in its whole length. 



. 41. SECTION or SCALP, showing two roots of hairs, a, a, 

 Blood-vessels; &,&, erector muscles of the hair; c,c, sebaceous 

 glands. 



