84 CONSCIOUS NERVOUS OPERATIONS 



give its color. There are also cavities filled with air. 

 These small bodies of inclosed air are very abundant in 

 white hair, and, reflecting the / light, are the cause of its 

 color, just as the whiteness of snow is due to the many 

 reflections of light from the tiny bits of ice which compose 

 it. 



In the nails the horny layer of the epidermis is greatly 

 developed. Each nail has its root firmly embedded in a 

 groove of the cuticle. The under side is fixed to the der- 

 mis except at the end of the tinger or toe, where there is 

 a free edge. 



The hair preserves the temperature of the head and 

 is also a protection against injury. The nails protect 

 the sensitive ends of fingers and toes and aid in their 

 mechanical operations. Both hair and nails also help in 

 the ornamentation of the body. 



108. Mucous Membrane. Not only is the whole outer 

 surface of the body covered with the skin, but all the 

 inner cavities and passages which have an external com- 

 munication are also lined with it. These inner linings 

 are, however, of a modified form of skin called mucous 

 membrane, because it secretes a viscous fluid, or mucus. 

 This membrane is thinner, redder, and more sensitive and 

 delicate than the outer skin, but is of the same general 

 composition ; that is, it has the two layers, the outer 

 bloodless and insensible, the inner highly sensitive, soft, 

 fully supplied with blood vessels, glands, etc. 



109. The Dermis, or True Skin, is a close network of 

 connective tissue fibers, forming a dense, tough, firm 

 envelope for the body, resting upon and gradually pass- 

 ing into the areolar tissue beneath, which is a loose 

 network of interlacing bands and cords with meshes 

 between. Ramifying through the derm is are nerves, 



