390 



CIRCULATION AND RESPIRATION 



Epidermi 



special forms. A hair is a tiny tube, formed in a pocket of 

 the dermis. The part projecting from the skin, like the 

 epidermis itself, has neither blood vessels nor nerves. But 



the inner end of the hair 

 is in the true skin, and 

 is kept growing by the 

 formation of new cells. 

 These are nourished by 

 the lymph of the dermis. 

 Tiny muscles in the hair 

 "pits" sometimes cause 

 the hair to " stand on 

 end, ' ' causing ' ' goose- 

 flesh." The hair covers 

 nearly the whole body. 



FIG. 292. 

 Structure of the Nails and the Hair. 



In the hair pits there are glands that secrete an oil for the hair; 

 these are the oil glands, or sebaceous glands. The oil softens the skin 

 and the hair. Brushing the hair helps the oil glands in their work, for 

 it spreads the oil along the outside of the hair. 



Dandruff in excess is generally considered a disease. It is prob- 

 ably due to bacteria, and may spread from one person to another. 

 Only clean combs and brushes should ever be used for the hair. If 

 we pull a hair out without destroying the cell that produces it, a new 

 hair will grow. In baldness the cells lose the power of producing new 

 hair. The cause of all baldness is not known, but much of it seems to 

 result from dandruff, or from a lack of fat in the scalp. 



The nails are plates of tough epidermis that grow out of 

 depressions in the dermis. They grow in length and 

 thickness as new epidermis cells are formed at their inner 

 ends, or roots. The older part of a nail is transparent; 

 the blood vessels beneath it make it appear pink. 



