CHAP. VIII.] EPITHELIAL TISSUES, &C. 297 



transparent than previously ; subsequently, the nucleus may disap- 

 pear, leaving the cell body. In some cases, the separate anatomical 

 elements are best seen if after the action of alkali the tissue is placed 

 in water. Cold concentrated sulphuric acid also brings out the cells 

 of the epidermis, effecting to a certain extent their dissociation. 

 When heated, it dissolves the deepest cells (those of the rete mucosum) 

 but leaves undissolved the more superficial, in which the cell protoplasm, 

 has undergone conversion into horny substance. 



structure of The Nails possess essentially the same structure as 



Nails. the epidermis, and their cells may, like those of the 



latter structure, be divided into an upper horny layer 

 and a lower softer stratum. The action of caustic potash or soda on 

 the cells of nail is similar to that exerted by these reagents on the 

 cells of the cuticle. 



structure of Horn is constituted exactly as nail ; in the deeper 



Horn. layers of cells pigment is sometimes present, as it is in 



the rete Malpighii of the epidermis. 



structure of Hoof is composed of compressed epithelial cells 



Hoof. arranged concentrically around canals which run from 



above downwards. The individual cells are made apparent by treat- 

 ment with solutions of caustic alkalies. 



structure of Hairs have a more complex structure than the 



Hair. epidermal tissues yet enumerated, and for a description 



we must refer to treatises on Histology. It may be mentioned 

 however that the stem of the hair is seen to be covered by layers of 

 imbricated cells which are evidently modified epidermic epithelial 

 cells ; within these is the so-called fibrous substance which makes 

 up the greater part of the stem and which may be resolved into 

 elongated scales; and in the very centre of the hair is the medulla, 

 or pith in which sometimes air spaces are seen, sometimes cells which 

 are filled with fatty and pigmentary particles. The lower part of the 

 stem or shaft of the hair dips into the so-called hair-follicle, where 

 it is attached to, or rather grows upon, the papilla ; the imbricated 

 scales which cover the root of the hair are continuous with the inner- 

 most layer of cells of the epidermic lining of the hair-follicle. 



Horny Substance or Keratin, 



By the term Keratin is understood the organic substance, or 

 perhaps the mixture of organic substances, left as an insoluble residue 

 when cuticle, horn, nails, hairs, feathers, &a, are successively boiled in 

 ether, alcohol, water, and dilute acids. This insoluble residue retains 

 the form of the tissues from which it is prepared; it is little 

 affected by boiling with water at ordinary pressure, but is dissolved 

 when subjected to the prolonged action of water under pressure (as 



