THE SKIN. 201 



make up the chief mass of the hair-shaft. In the centre of this 

 mass there is sometimes a line of more loosely aggregated cells, 

 forming the " medulla " of the hair. When this is present the sur- 

 rounding part of the shaft, between it and the cuticle, is known as 

 the "cortex" (Figs. 177 and 178). 



The sebaceous glands (Fig. 173, d) are sacculations in the corium 

 near the hair-follicles, which are filled with epithelial cells. The 

 cells at the periphery divide, and, as they increase in size, push 



FIG. 178. 



Hair-follicle from the human scalp. (Mertsching.) Cross-section from middle third of 

 the follicle: 6, longitudinal and encircling layers of the fibrous coat; c, hyaline layer, 

 formed of an outer faintly fibrillated and an inner more homogeneous lamina, d ; e, outer 

 root-sheath, continuous with rete mucosum of epidermis ;/, Henle's sheath; g, Huxley's 

 layer; h, cuticle of root-sheath; k, cuticle of hair; I, cortical cells of the hair; ra, 

 medulla. 



each other toward the centres of the sacs. Here they undergo a 

 fatty degeneration, ending in destruction of the cells and the forma- 

 tion of an oily secretion, the sebum, which is discharged into the 

 hair-follicle a short distance below its opening on the surface of the 

 skin. The sebum is a lubricant for both the hair and the epi- 

 dermis (Fig. 179). 



The color of the epidermis and of the hair is due to a pigmenta- 

 tion of the cells in the deeper layers of the rete mucosum and those 

 composing the hair. The whiteness of the hair which comes with 

 years is due to little spaces which appear in unusual numbers 

 between the cells of the cortex, and are filled with air, reflecting the 

 light and masking the pigmentation of the cells. 



The nails are especially thick and condensed masses of epithelial 

 cells which have undergone keratoid degeneration and are closely 

 compacted. They are produced at the root of the nail, and as they 



