572 



THE SKIN. 



much as through their contraction a narrow neck is estab- 

 lished around the young hair, as first suggested by Biesiadecki. 

 (See Fig. 238.) 



The Development of the Hair can be first traced at the end of 

 the third month of intrauterine life. The epithelial investment 

 of the skin produces a knob-like prolongation downward, while 

 later an extension of the connective tissue is formed, which 

 lifts the bottom of the epithelial knob, and produces the papilla. 

 The epithelia are originally all medullary in character, and from 

 the medullary corpuscles, by elongation and mutual flattening, 



FIG. 239. SCALP OF MAN. VERTICAL SECTION. 



R, root of hair ; IS, inner root-sheath ; OS, outer root-sheatli ; G, sebaceous gland ; D, duct 

 of the sebaceous gland; A, acarus lolliculorum within the duct of the sebaceous gland. 

 Magnified 200 diameters. 



arise the elements composing the hair in the center of the knob, 

 and also the root-sheaths at its peripheral portion. 



(9) The' Sebaceous Glands. The acinous sebaceous glands are 

 formed from the outer root-sheath of the hair, and are usually 

 in close relation to the hair. Sebaceous glands without hair 

 are found in the areole around the nipple of the female breast, 

 in the glans and the prepuce of the penis, in the nymphse and 



