NAILS AND HAIR. 123 



varies with their thickness, but an ordinary hair from the 

 head will bear a weight of six or seven ounces. A well- 

 known property of the hair is that of becoming strongly 

 electric by friction ; and this is particularly well marked 

 when the weather is cold and dry. The electricity thus 

 excited is negative. Sections of the shaft of the hairs show 

 that they are oval, but their shape is very variable, straight 

 hairs being nearly round, while curled hairs are quite flat. 

 Another peculiarity of the hairs is that they are strongly hy- 

 grometric. They readily absorb moisture and become sen- 

 sibly elongated, a property which has been made use of by 

 physicists in the construction of delicate hygrometers. 



Hoots of the Hairs and Hair -follicles. The roots of the 

 hairs are embedded in follicular openings in the skin, which 

 differ in the different varieties only in the depth to which 

 they penetrate the cutaneous structure. In the downy hairs, 

 the roots pass only into the superficial layers of the true 

 skin ; but in the thicker hairs, the roots pass through the 

 skin and penetrate the subcutaneous cellulo-adipose tissue. 



The root of the hair is softer, rounder, and a little larger 

 than the'shaft. It becomes enlarged into a rounded bulb at 

 the bottom of the follicle, and rests upon a f ungiform papilla, 

 constricted at its base, to which it is closely attached. In 

 describing the connection between the hairs and the skin, 

 anatomists mention three membranes forming the walls 

 of the hair-follicles, and two membranes that envelop the 

 roots of the hair in the form of a sheath. The study of these 

 parts is much simplified by keeping constantly in view the 

 correspondence between the different layers of the follicles 

 and the layers of the true skin, and the relations of the root- 

 sheaths with the epidermis. 



The follicles are tubular inversions of the structures that 

 compose the coriurn, and their walls present three distinct 

 membranes. Their length is from -^ to J of an inch. The 

 membrane that forms their external coat is composed of 



