262 



THE TISSUES, 



166. 



Development of the Hairs. 



Hairs are first seen in the foetus, upon the forehead and eyebrows, 

 from the third to the fourth month ; the first rudiments of the hair- 

 sacs consisting of flask-shaped 

 solid processes of the Malpighian 

 layer of the epidermis, formed 

 by its growth inwards. (Fig. 166.) 

 The internal cells of these be- 

 come converted into a delicate 

 hair, surrounded by its internal 

 root-sheath; and the external still 

 remaining soft, constitutes the 

 outer root-sheath and the hair- 

 bulb. Thus the hairs, unlike the 

 teeth, arise at once in their to- 

 tality. (Kolliker.) The first hairs 

 are formed merely of elongated 

 cells similar to the fibrous sub- 

 stance of the later hairs, the me- 

 dullary cells being entirely ab- 

 sent. The cuticle is, however, 

 clearly visible. The papilla is to 

 be regarded as an outgrowth of 

 the fibrous layer of the hair-sac, 

 analogous to the papillas of the 

 skin. 1 (Kolliker.) . j 



The hairs themselves never 

 appear under from three to five 



weeks after the rudiments just described, e. g. at the nineteenth 

 week, the hairs themselves are nowhere to be seen except on the 

 forehead and eyebrows; and, in the twenty-fourth week, they are 



1 The translators of Kolliker's work (Drs. Busk and Huxley) adopt Reichert's 

 view of the development of the hair, viz., that it results from the cornification of 

 a dermic papilla, and regard a hair as homologous with a tooth, in all its parts. 

 " The substance of the shaft corresponds with the dentine, offering even rudiment- 

 ary tubes in its aeriferous cavities ; the inner layer of the cuticle answers to the 

 enamel, the outer to Nasmyth's membrane, and whoever will compare these struc- 

 tures will be struck by the similarity even in their appearance. The sac answers 

 to the dental capsule ; the outer root-sheath to the layer of epithelium (enamel 

 organ) next the capsule ; the fenestrated membrane to the stellate tissue, and what 

 Professor Kolliker calls { Huxley's layer,' to the columnar epithelial layer of the 



A. Kudiments of hair-sacs I, I (foetus, 16 weeks). 

 B. Single hair-sac seen laterally, a, &. Cuticle 

 and stratum MalpigMi of the skin. i. Basement- 

 membrane of hair-sac prolonged from between 

 the stratum Malpighii and the corium. m. Bound- 

 ed and elongated cells forming the matrix of the 

 hair. 



