KIDNEYS, SKIN, AND GENERAI, EXCRETION. 405 



hair follicle. By their contraction it is probable that a lit- 

 tle of this sebaceous secretion is squeezed out of the gland 

 into the follicle. These muscles are specially apt to con- 

 tract under certain conditions such as exposure to the cold, 

 and by this contraction the follicle is pulled upward and 

 projects slightly beyond the rest of the epidermis, producing 

 the little elevations described under the somewhat senseless 

 term of goose-skin. 



These glands as well as hairs are absent from certain 

 portions of the skin such as the palms of the hands and the 

 soles of the feet. A very specialized form of such a sebaceous 

 gland is found among the tail feathers of certain birds and is 

 called the uro-pygeal gland. The secretion here is so 

 abundant that the bird in question may by means of its bill 

 take the secretion for the oiling of its entire coat of feath- 

 ers. A special form of these glands in the human body is 

 found along the upper and lower eyelids. These glands are 

 known as the Meibomian glands. The function of the se- 

 cretion at this point is to keep the edge of the lids some- 

 what oily and so prevent the tears from running out of the 

 eye over these eyelids. This is accomplished by taking 

 advantage of the familiar fact that water does not very 

 readily run across an oiled surface. In the ear there are 

 found certain sebaceous glands secreting a thick, fatty sub- 

 stance familiar as the ear-wax. These glands, however, 

 are anatomically not true sebaceous glands at all, in spite 

 of the nature of their secretion, but belong to the tubular 

 sweat glands. It is in fact just possible that these are not 

 modified sebaceous glands at all, but are really sweat glands 

 which have taken on a different function. 



THE SWEAT GLANDS. 



Distributed all over the body, except on the palms of 

 the hands and on the soles of the feet, are long tubular 

 glands known as the sweat or sudoriferous glands. These 

 are simple tubular glands, the lower portion of the tube, 

 however, being rolled up into a coil. This coil lies in the 



