^28 THE CELL AND MAMMALIAN HISTOLOGY 



especially in mammals such as rodents, the dermis contains 

 smooth muscle, and in the skin of the cat and in that of the face 

 of man there is voluntary muscle. Hairs appear to take their 

 origin from the dermis, but are in fact epidermal structures. 

 A downgrowth from the surface forms a folHcle, and from the 

 base of this the hair grows up, so that it projects above the 

 surface. Into the base have grown blood vessels and other tissues 

 of the dermis to form a papilla. A hair is made of many cells, 

 typically arranged to form a cylinder round a softer core. It 

 consists largely of keratin, and has a variable amount of pigment. 

 In old age the core may break down and become filled with air 

 bubbles, which give a white appearance. Opening into the hair 

 follicles are sebaceous glands, which are derived from the stratum 

 of Malpighi and break down to form a mixture of hpoid material 

 and ceil debris called sebum. This makes a waterproof and anti- 

 septic film which is squeezed over the skin when the smooth 

 muscles attached to the hairs contract. Its lipoid part, which 

 consists mostly of esters, not of glycerol but of higher alcohols 

 such as cholesterol, is well known in the form of wool-wax or 

 lanohne, which is the chief constituent of face-creams and brush- 

 less shaving cream ; a similar material is found in the preen glands 

 of birds. On exposure to sunlight some of these esters are converted 

 to Vitamin D, and in some animals licking of the fur may be an 

 important source of this. 



Sweat glands are long tubes, of which the deeper and secretory 

 parts are coiled in the lower part of the dermis, or, on the palms 

 and soles, below this in the superficial fascia. Their secretion 

 is more than ninety-nine per cent, water, with a little sodium 

 chloride and negligible amounts of urea and other organic 

 substances, so that they are of no importance in excretion. The 

 function of the sweat glands is to control the temperature of the 

 body by providing water which can evaporate, and it is only 

 when the skin temperature has been raised above normal by 

 exercise or heat that visible sweating begins. 



The structure of the skin of the frog, which is without hairs and 

 sweat glands, but possesses other glands of its own, is shown in 

 Fig. 262. The scales of reptiles are thickenings of the epidermis, 

 and feathers are comparable in their origin to hairs. Claws, nails, 

 hooves and horns are also epidermal structures. 



