322 THE SKIN 



granules and press toward the lumen of the alveoli. This process is in 

 essential respects like that of the goblet-cells secreting mucin. Some- 

 times the cell does not go to pieces in situ but only after being extruded 

 from the gland. The excretory ducts are wide and ordinarily empty 

 into the upper third of the hair-follicle; their walls also secrete sebum. 



The functions of sebum are not hard to understand when its general 

 fatty, emollient nature and the place of its excretion are kept in mind. 

 It is essentially and primarily an oily softener of the epidermis and of the 

 hairs found nearly everywhere over the body. Without it the epidermal 

 scales and the scaly hair would become hard and brittle and unfit to 

 perform their protective functions. One of the most common features 

 of the body's environment is water or watery vapor. The fat which 

 the sebum supplies to the epidermis serves to make the latter tough and 

 impervious to water, which else sometimes would be absorbed by the 

 capillaries and derange the organism more or less. The hairs especially 

 are dependent on an abundance of oil for their normal suppleness and 

 strength. In general over the body then the sebum keeps the skin's 

 outer layer soft and pliable, and prevents the maceration of the epidermis 

 which water would else be sure to cause. There are no sebaceous 

 glands on the palms of the hands or on the soles of the feet, nor in two 

 or three other narrow localities. 



In special locations, mostly mentioned above, the sebum has special 

 functions in addition to its general uses just described : In the eyelids, the 

 secretion of the sebaceous (Meibomian) glands has the function of pre- 

 venting the cohesion of the lids when they are closed, as during sleep. 

 One sees its use in conjunctivitis of various sorts, for then the secretory 

 process is often exaggerated and the lids cannot be opened in the morning 

 until the adhesive sebum is softened and removed. 



In the inner part of the external ear, the external meatus, the sebum is 

 termed cerumen or ear-wax. There it lubricates the membrana. It 

 serves also the purpose of partly preventing the entrance of insects and 

 even of small particles of lifeless foreign matter, the stiff hairs also often 

 present aiding in this. For the purpose of making the cavity still more 

 inhospitable to insects, the cerumen has an intensely bitter taste, thus 

 the better preventing small insects from coming in contact with the 

 membrana tympani. 



On the lips and corners of the mouth the sebum is necessary to prevent 

 the cracking of the integument, for these are places where the skin is 

 thin and liable to be broken or slightly torn. 



The vernix caseosa which covers the infant at birth is evidently a 

 lubricant facilitating the passage outward of the child and preventing 

 abrasion both of the latter's very delicate skin and of the birth-canal of 

 the mother. Pigeons' milk is a form of sebum on which the young birds 

 are fed at first; it conies from temporary sebaceous glands developed in 

 the crops of both parents. The sebum of sheep is called lanolin, and 

 serves to keep the wool soft, pliable, and water-proof. When the amount 

 of the sebum is generally excessive in quantity the condition is known 



