THE HAIR. 



263 



not yet developed on the hand and foot, and on some parts of the 

 forearm and leg. In some 

 parts, the hairs penetrate 

 the epidermis as soon as 

 formed (on the eyebrows 

 and eyelashes); in others, 

 their points engage ob- 

 liquely between the pro- 

 per epidermis and the Mai - 

 pighian layer, and grow 

 for a time under the for- 

 mer as on the chest, ab- 

 domen, back, and extre- 

 mities. The lanugo is fully 

 developed in the twenty- 

 third to the twenty -fifth 



Development of hair (eyebrows). A. First separation of 

 inner and outer portions of the matrix. B. First formation 

 Week. It gradually aC- O f the hair, the point not yet appearing above the skin. c. 



Hair seen after its emersion, a. Cuticle of skin. b. Stra- 

 tum Malpighii of do. c. Onter root-sheath, d. Inner root- 



quires a darker color; be- 

 coming even almOSt black sheath, e. Bulb. /. Shaft, g. Point of the hair. h. Pa- 

 On the head, before birth pilla " * Basement-membrane, n, n. Commencement of 



the sebaceous glands. 



in some cases. A small 



portion, however, falls off, is swallowed by the foetus with the liquor 

 amnii, and thus appears in the meconium. Fig. 167 shows the pro- 

 gress of the development of the hairs. 



The hairs are shed after birth, new ones forcing out and taking 

 the place of the old ones. The new are formed in processes 

 shooting off from the original hair-sacs. Kolliker discovered and 

 first propounded this law, though it is not yet certain that all the 

 hairs fall out, nor at what precise period after birth. The hairs of 

 the head in many children are known to fall out within the first 

 two to six months. The stages of development of the new hairs, 

 as well as the relation of the hairs to the sebaceous follicles already 

 alluded to (p. 227), is shown by Figs. 168 and 135. 



The periodical shedding of the hairs of the lower animals is pro- 

 bably secured in the same way ; new ones being formed in the old 



organon adamantince" (p. 190). Without precisely adopting all the analogies just 

 quoted, we admit that a hair is very analogous to a tooth ; for we regard a tooth as 

 well as a hair, as being essentially an epidermic (epithelial) production. The 

 manner in which the hairs fall out, and are succeeded by others, shows (as well as 

 their development), that they are the analogues of the teeth on the one hand, and 

 of the epidermis on the other. 



