396 



HISTOLOGY 



fied, elongated and very closely joined together. Their nuclei are then 

 linear. The cortex of colored hairs contains pigment both in solution 



and in the form of granules. These 

 granules are partly within the cells, 

 and partly between them. Moreover 

 every fully developed hair contains 

 minute intercellular air-spaces, found 

 within both cortex and medulla. But 

 a medulla is lacking in many hairs, and 

 when present, in the thicker hairs, it 

 does not extend their whole length. It 

 consists of cuboidal cells containing 

 kerato-hyalin (Fig. 399), and generally 

 arranged in a double row. Their nu- 

 clei are degenerating. 

 Growth and Replacement of Hairs. The growth of the shaft, and of the 

 inner epithelial sheath with its cuticula, takes place through continued 



Remains of inner 

 sheath. 



Epithelial bed. 



FIG. 406. FOUR STAGES IN THE SHEDDING OF 

 A HAIR. FROM THE SKIN OF THE NOSE 

 OF A SEVEN AND ONE-HALF MONTHS 

 EMBRYO. X 50. 



Parts of A and B are shown enlarged in Figs. 

 407 and 408. 



Cornified bulb. 



/Remains of inner 

 sheath. 



H %;<* y, 



&& 



Epithe 



\L f -l ** 



Pi bS al 



.V*AV 



Thick hyaline -_ . 

 membrane. 



Epithelial ' 

 cord. 



Matrix cells. 

 Papilla. 



Epithelial cord. 



Atrophic papilla. 

 Connective tissue. 



PIG. 407. LOWER PART OF FIG. 406, A. X23O. FIG. 408. LOWER PART OF FIG. 406, B. X 230. 



mitotic division of the epithelial matrix cells of the bulb of the hair. These 

 become cornified, and are added from below to the cells previously cor- 

 nified. Accordingly the oldest cells are at the tip of the hair and the young- 



