TACTILE SENSE. 



735 



obliquely from the skin. The hairs of the white race are cylindrical ; 

 the hair of the negro flattened cylindrical. In structure the hairs 

 consist of an exterior cuticle, a cortex, and an interior medulla. 

 The cuticle consists of a single layer of thin, colorless, quadrilateral 

 scales which overlap like the shingles of a roof. The edges of the 

 scales are directed upward and outward along the shaft. The cortex 

 makes the chief part of the hair, and it is that upon which the color 

 of the hair mainly depends in different individuals. The cortical 

 layer is made up of elongated, fusiform cells containing a lineal 

 nucleus. When the coloring matter disappears in the cortex the hair 

 becomes white. The medulla is frequently absent, especially in the 

 dark-colored hairs. It occupies the axis of the hair. It consists of 

 cuboidal cells with granular contents and an indistinct nucleus. The 

 medullary substance is generally mingled with more or less air, in 

 small bubbles, which penetrates from the ends of the hairs and gives 

 to these when white the characteristic silver luster. The root of the 

 hair is lodged in a flask-shaped receptacle of the skin called the hair- 

 follicle, at the bottom of which is a papilla from which the hair 

 grows. "Goose-flesh" is due to minute muscles contracting and 

 causing the hair-follicles to become erect. At the same time the 

 sebaceous glands are compressed, favoring the exudation of the seba- 

 ceous secretion. 



Chemically, the hairs are mainly composed of an albuminoid 

 derivative, keratin, in which a notable quantity of sulphur is present: 

 about 5 per cent. In the ashes are found the phosphates, earthy sul- 

 phates, oxide of iron, and pigment. 



FUNCTION. The large hairs serve to protect the skin, breaking 

 shocks and preventing a considerable loss of heat. In other places, 

 like the armpits, they prevent friction and attrition of the skin 

 layers. The downlike hairs render the touch more delicate. 



Nails. The nails are hard appendages of the skin, and corre- 

 spond to the claws of animals. They are flexible, translucent, 

 square-shaped plates continuous with the epiderm and resting on a 

 depressed surface of the dermis called the matrix, or bed. 



The exposed part of the nail is the body and its anterior end is 

 its free border. The root of the nail is lodged in a deep groove of 

 the matrix and the lateral borders are received into shallow grooves. 

 The half -moon, or lunule, of the nail is due to a less degree of vascu- 

 larity of the matrix at the root, defined by a semicircular line. The 

 horny layer corresponds to the cuticle of the epiderm, and is com- 

 posed of flattened, nucleated cells. The soft layer of the nails, the 



