198 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



pies 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, albuminous 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 neighboring sweat-glands or ceruminous glands, 

 forms the well-known ear-wax or cerumen. The secretion in this place con- 

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

 not been investigated. Upon the skin of the newly-born the sebaceous ma- 

 terial is accumulated to form the vernix caseosa. The well-known uropygal 

 gland of birds is homologous with the mammalian 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 satu- 

 rated with external moisture. In this way it probably 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 which tends to prevent undue loss of heat from evaporation, and possi- 

 bly is important in other ways in maintaining the physiological integrity of 

 the external surface. 



Sweat. The sweat or perspiration is a secretion of the sweat-glands. 

 These latter structures are found over the entire cutaneous surface except in 

 the deeper portions of the external auditory meatus. They are particularly 

 abundant upon the palms of the hands and the soles of the feet. Krause 

 estimates that their total number for the whole cutaneous surface is about two 

 millions. In man they are formed on the type of simple tubular glands ; the 

 terminal portion contains the secretory cells, and at this part the tube is 

 usually coiled to make a more or less compact knot, thus increasing the extent 

 of the secreting surface. The larger ducts have a thin muscular coat of invol- 

 untary tissue which may possibly be concerned in the ejection of the secretion. 

 The secretory cells in the terminal portion are columnar in shape, they possess 

 a granular cytoplasm and are arranged in a single layer. The amount of 

 secretion formed by these glands varies greatly, being influenced by the con- 

 dition of the atmosphere as regards temperature and moisture, as well as by 

 various physical and psychical states such as exercise and emotions. An aver- 

 age quantity for twenty -four hours is said to vary between 700 and 900 grams, 

 although this amount may be doubled under certain conditions. 



Composition of the Secretion. The precise chemical composition of sweat 

 is difficult to determine, owing to the fact that as usually obtained it is liable 



