258 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



terial is accumulated to form the remix caseosa. The well-known u ropy gal 

 gland of birds is homologous with the mammalian sebaceous glands, and its 

 secretion lias 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 that 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 that 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. The 

 average quantity for twenty-four hours is said to vary between TOO and 900 

 grams, although this amount may be doubled under certain conditions. 



According to an interesting paper by Schierbeck, 1 the average quantity 

 of sweat in twenty-four hours may amount to 2 to 3 liters in a person clothed, 

 and therefore with an average temperature of 32° C. surrounding the skin. 

 This author states that the amount of sweat given off from the skin in the 

 form of insensible perspiration increases proportionately with the tempera- 

 ture until a certain critical point is reached (about 33° C. in the person 

 investigated), when there is a marked increase in the water eliminated, the 

 increase being simultaneous with the formation of visible sweat. At the same 

 time there is a more marked and sudden increase in the CO., eliminated from 

 the skin, from 8 grams to 20 grams in twenty-four hours. It is possible that 

 the sudden increase in (X )., is an indication of greater metabolism in the sweat- 

 glands in connection with the formation of visible sweat. 



Composition oftht Secretion. — The precise chemical composition of sweat 

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

 1 Archivfur Anatomieund Physiologie (Physiol. Abtheil), 1893, S. llfi. 



