770 PHYSIOLOGY OF DIGESTION AND SECRETION. 



from the layer nearest the basement membrane, and thus the glands 

 continue to produce a slow but continuous secretion. The sebaceous 

 secretion, or sebum, is an oily, semiliquid material that sets, upon 

 exposure to the air, to a cheesy mass, as is seen in the comedones 

 or pimples which so frequently occur upon the skin from occlusion 

 of the opening of the ducts. The exact composition of the secretion 

 is not known. It contains fats and soaps, some cholesterin, albu- 

 minous material (part of which is a nucleo-albumin often described 

 as a casein), remnants of epithelial cells, and inorganic salts. The 

 cholesterin occurs in combination with a fatty acid, and is found in 

 especially large quantities in sheep's wool, from which it is extracted 

 and used commercially under the name of lanolin. The sebaceous 

 secretion from different places, or in different animals, is probably 

 somewhat variable in composition as well as in quantity. The 

 secretion of the prepuce is known as the smegma prceputii; that of 

 the external auditory meatus, mixed with the secretion of the neigh- 

 boring sweat glands or ceruminous glands, forms the well-known 

 earwax or cerumen. The secretion in this place contains a reddish 

 pigment of a bitterish-sweet taste, the composition of which has 

 not been investigated. Upon the skin of the newly born the se- 

 baceous material is accumulated to form the vernix caseosa. The 

 well-known uropygal gland of birds is homologous with the mam- 

 malian sebaceous glands, and its secretion has been obtained in 

 sufficient quantities for chemical analysis. Physiologically it is 

 believed that the sebaceous secretion affords a protection to the 

 skin and hairs. Its oily character doubtless serves to protect the 

 hairs from becoming too brittle, or, on the other hand, from being 

 too easily saturated with external moisture. In this way it prob- 

 ably aids in making the hairy coat a more perfect protection against 

 the effect of external changes of temperature. Upon the surface of 

 the skin, also, it forms a thin, protective layer that tends to prevent 

 undue loss of heat from evaporation, and possibly is important in 

 other ways in maintaining the physiological integrity of the external 

 surface. 



Excretion of CO 2 . In some of the lower animals the frog, 

 for example the skin takes an important part in the respiratory 

 exchanges, eliminating CO 2 and absorbing O. In man, and pre- 

 sumably in the mammalia generally, it has been ascertained that 

 changes of this kind are very slight. Estimates of the amount of 

 CO 2 given off from the skin of man during twenty-four hours vary- 

 greatly, but the amount is small, about 7 to 8 gms. in twenty-four 

 hours, unless there is marked sweating, in which case the amount is 

 noticeably increased. 



