THE FRAMEWORK OF THE BODY 51 



Some parts of the body have the skin very firmly attached to the 

 deeper tissues, but in other regions the skin is very loosely and movably 

 attached, permitting free play of the enclosed parts. The skin likewise 

 varies greatly in thickness from one region of the body to another. On 

 the inner surfaces of the hands and the soles of the feet, there are multi- 

 tudes of tiny ridges that increase friction and reduce the tendency to 

 slip. These form the familiar fingerprint pattern and leave their traces 

 on objects touched because of the alignment of sweat glands along the 

 ridges. The pattern of these ridges is individually characteristic and 

 remains constant throughout life. 



Appendages of the skin. The appendages or accessory structures 

 of the skin are the nails, hair, glands, and teeth. An inspection of Fig. 3.13 

 will show a hair follicle and root, a sweat gland and an oil gland. It will 

 be observed that both the hair follicle and the glands are formed by long 

 invaginations of the malpighian layers of the epidermis, extending deep 

 into the dermis. Incidentally, the sweat and oil glands show two rather 

 different devices by which an epithelium may be enormously expanded to 

 form a many-cell-powered secreting structure. 



Hairs are formed by the cells that line the bottoms of the deeply pitted 

 hair follicles. The part of the hair contained within the pit is the root; 

 the portion extending above the surface of the skin is the shaft. With the 

 exception of the palm of the hand, the sole of the foot, and the last 

 phalanges of the fingers and toes, the whole skin is provided with hair 

 follicles. The nature and color of the hair in different individuals are due 

 to details of hair growth and development that are, in part, determined by 

 inherited factors. In general, hair "grows" from the bottom of the follicle, 

 the previously formed portion being pushed out of the follicle to project 

 farther and farther from the surface of the skin. Loss of hair may be due 

 to inherited factors or to disease and functional disorders. 



Nails are composed of clear, horny dead cells joined to form a solid 

 continuous plate. Nail growth is rather similar to that of hair, except 

 that in the case of the nails, the nail-producing cells are densely packed 

 along the invaginated furrows from which the nails are developed. 



Oil glands occur everywhere over the surface of the skin except on 

 the palms of the hands and the soles of the feet. They are very abundant 

 in the scalp and in the face. Nearly everywhere they are associated with 

 hair follicles. A typical oil gland is shown in Fig. 3.13, opening into the 

 upper part of the hair follicle, through which its oily secretion reaches 

 the surface of the skin to spread as a protecting oily film. This film of oil 

 keeps hair from becoming dry and brittle and serves to prevent undue 

 absorption or evaporation of water from the skin itself. 



Sweat glands are abundant over the whole body skin but are largest 

 and most numerous under the arms, on the palms of the hands and the 



