404 STUDIES IN ADVANCED PHYSIOLOGY. 



On cross sections the shaft of the hair is usually round. 

 In some individuals, however, and in certain races it is 

 more regularly flattened. Such hairs show a natural tend- 

 ency to curl on account of the unequal growths at the different 

 angles. The size of these hair shafts is also a racial char- 

 acteristic. It is rather small in the Caucasian race, much 

 larger in the Mongolian, and reaches possibly its maximum 

 diameter in the American Indian and the Japanese. 



It need not be pointed out that like the nails and epider- 

 mis, hairs are able to be replaced only when the reproduc- 

 ing Malpighian layer at the root of the hair is left intact. 



Sebaceous Glands. Along the tube of the hair follicle 

 in which the shaft is immersed in the skin there opens reg- 

 ularly a sebaceous duct carrying a fatty secretion from the 

 sebaceous gland into the follicle, there to be poured upon 

 the shaft of the hair. This sebaceous gland is a flask-like 

 outgrowth from the epidermal cells of the follicle, and is 

 therefore epidermal in its origin. The function of this se- 

 cretion is not only to preserve somewhat the vitality of the 

 hair, but by being poured upon the skin to keep that in 

 pliable condition and to protect the same from excessive 

 evaporation. The secretion from these sebaceous glands is 

 of a fatty nature. In places where the hairs are minute 

 these sebaceous glands seem to open directly on the surface 

 of the skin, an arrangement especially noticeable on the 

 skin of the face. The secretion of such glands may fre- 

 quently become thickened and so choke up the glands in 

 question, a condition which usually results in the formation 

 of a pimple, which is nothing more than an attempt of na- 

 ture to empty the gland by a process of suppuration. Some- 

 times the mouth of the gland becomes filled with particles 

 of dust or dirt pressed down into them, and so gives rise 

 to the familiar blackhead. These glands seem to be emp- 

 tied as a rule by the contraction of muscles which are found 

 attached to each hair follicle. These muscles are bands of 

 plain muscular tissue attached at one end to the under sur- 

 face of the corium, at the other to the lower portions of the 



