HAIR. 



Epidermis. 



Epithelial column. 



boiling a fragment placed in a strong solution of caustic potash. The 

 cells retain their nuclei as is seen in Fig. 364. The forward movement 

 of the nail is due to the production of new cells from behind. 



HAIR. 



The hairs arise as local thickenings of the epidermis. They soon 

 become round columns of ectodermal cells extending downward into the 

 corium (Fig. 365). As the columns elongate the terminal portion becomes 

 enlarged, forming the bulb of the hair, and a mesodermic papilla occupies 

 the center o the bulb. On that side of the epithelial column which from 

 its obliquity may be called the lower surface, there are found two swell- 

 ings (Fig. 366 and 368). The outer is to become a sebaceous gland dis- 

 charging its secretion into the epithelial column; the inner or deeper 

 swelling is called the hair matrix and 

 its cells, which increase by mitosis, 

 contribute to the growth of the col- 

 umn. (The lower swelling is often 

 described as the place of insertion of 

 the arrector pili muscle.) Beginning 

 near the bulbus the core of the column 

 separates from the peripheral cells; 

 the latter become the outer sheath of 

 the hair. The core forms the inner 

 sheath and the shaft of the hair. The 

 cells of the shaft become cornified just 

 above the bulbus, and they are sur- 

 rounded by the inner sheath as far 



as the sebaceous gland. Beyond this point the inner sheath degen- 

 erates so that in later stages the distal part of the shaft is immediately 

 surrounded by the outer sheath. As new cells are added to the 

 hair from below, the shaft is pushed toward the surface. The central 

 cells in the outer end of the column degenerate, thus producing a "hair 

 canal" which is prolonged laterally in the epidermis (Fig. 369). The 

 shaft enters the canal, breaks up the overlying epitrichium, and projects 

 from the surface of the body (Fig. 370). That portion of the hair which 

 remains beneath the epidermis is called its root. In addition to the epi- 

 thelial sheaths, the root of all larger hairs possesses a connective tissue 

 sheath derived from the corium. This serves for the insertion of a bundle 

 of smooth muscle fibers which arise in connection with the elastic elements 

 of the superficial part of the corium. Since this muscle by contraction 

 causes the hair to stand on end it is called the arrector pili. Its insertion 



Mesenchyma. 



Mesenchyma. 



FIG. 365. VERTICAL SECTION OP THE SKIN 

 OF THE BACK OF A HUMAN FETUS OF 

 FIVE MONTHS. X 230. 



