HAIR 



391 



Blood vessel. 



Hair canal. 



Epithe- 

 lium. 



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 bulb, 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 bulb, and 

 they are surrounded by the inner sheath as far as the sebaceous gland. 

 Beyond this point the inner sheath degenerates, 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 degen- 

 erate, thus producing a 

 "hair canal" which is pro- 

 longed laterally in the epi- 

 dermis (Fig. 397). The 

 shaft enters the canal, 

 breaks up the overlying 

 epitrighium, and projects 

 from the surface of the 

 body. That portion of the 

 hair which remains be- 

 neath the epidermis is 



- -"- 

 - 



--.-"- Degenerating inner 

 *^"-C sheath. 



Epithelial bed. 



I ',." , Outer sheath. 



called its root, 

 tion to the 



In addi- 

 epithelial 



FIG. 397. VERTICAL SECTION OF THE SKIN OF THE BACK'OF A 



HUMAN EMBRYO OF FIVE AND A HALF MONTHS. X 120. 



The staining with iron hsematoxylin has made the horny parts so 



black that their details are invisible. 



sheaths, the root in all 

 larger hairs possesses a connective tissue sheath, derived from the corium. 

 This serves for the insertion of a bundle of smooth muscle fibers, the other 

 end of which is connected with the elastic and fibrous elements in the super- 

 ficial part of the corium. Since this muscle by contraction causes the 

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

 below the sebaceous gland and on the lower surface of the hair, as shown 

 in Fig. 398. The hairs which cover the body of the embryo, persisting 

 after birth to a variable extent, are soft and downy, and are known 

 as lanugo. Arrector muscles are absent from the lanugo of the nose, 

 cheeks and lips, and also from the eyelashes (cilia) and nasal hairs 

 (vibrissae) . 



Adult Structure. The general appearance of hairs in sections of the 

 adult skin is shown in Fig. 398, which includes also the sebaceous glands 

 emptying into the sheaths of the hairs, and sweat glands which are usually 



