734 PHYSIOLOGY. 



in which their excretory duct ends. Their size is, in general, in- 

 verse to the volume of the corresponding hair-follicle. Where the 

 hairs are large the sebaceous glands seem to be appendages, and 

 when the hairs are small its hair-follicle seems to be an appendage 

 of the sebaceous gland. The glands are aciniform, surrounded by 

 a thin, connective tissue with a basement membrane studded with 

 epithelial cells infiltrated with fat, and the cells are more fatty in 

 the direction of the excreting duct, where is found free fat, due to 

 the destruction of the cells. When the sebaceous secretion stagnates, 

 it forms a fatlike mass which, when expressed, as in the nose, forms 

 the comedo, a wormlike body. The black-heads, as they are called, 

 are dirt in the surface of the gland. When the comedo is pressed 

 out of the duct it has been mistaken as the head of the worm. The 

 sebaceous matter contains, even in healthy individuals, the pimple- 

 mite, or Demodex folliculorum. 



There are three varieties of sebaceous secretions: (1) the seba- 

 ceous secretion proper of the skin, (2) the vernix caseosa of the new- 

 born child, and (3) the smegma of Tyson's glands of the prepuce. 



Function. The sebaceous matter anoints the hairs with oil in 

 their progress of growth from the skin. The greasiness of the sur- 

 face of the skin caused by this secretion permits the dust readily to 

 adhere, which makes soap necessary to remove its excess. Seba- 

 ceous secretion is made up of olein, palmitin, cholesterin, and earthy 

 phosphates. 



The organ of touch is also protected by the horny layer of the 

 epidermis, whose cells are being constantly removed by friction and 

 as constantly renewed by proliferation of the cells of the cutis vera. 



The modifications of the epidermis in man are the hair and the 

 nails. 



Hair. The hairs are threadlike appendages to the skin project- 

 ing from almost every part of its surface except the palms and soles. 

 They are flexible, elastic, and shining, but vary in degree of develop- 

 ment, fineness, color, and form in different races and the sexes as 

 well as in different persons. The color of the hair varies from a 

 light color to a black. The black hairs are found in all parts of the 

 globe and in all latitudes, as in the Esquimaux, negro, Indian, and 

 Malay. All the colored races have black hair, and this is true in 

 some groups of the white race. Red hair is represented in al] races. 

 The hair is composed of a projecting part, the stem, terminated by 

 the point, or end. The portion inserted into the skin is the root, 

 which begins in a clublike expansion. The hairs generally project 



