192 THE MICROSCOPIST. 



into an external and internal portion. The cells of the 

 hair may be isolated by sulphuric acid or solution of soda. 

 They overlap each other like tiles, so as to present undu- 

 lating or jagged lines across the surface of a fresh hair. 

 The felting property of wool depends on the looseness of 

 this overlapping. Air-bubbles are often found in hair, 

 especially in the medullary or axial portion, and give a 

 silvery appearance to white hair. The granules of pig- 

 ment are generally found in the cortical portion. 



Nails are nothing more than modified cuticle, depen- 

 dent for their growth -on the vessels of the matrix or bed 

 of the nail. Their epithelial cells may be demonstrated 

 by soaking in caustic soda or potash. 



Corns, icarts, and horn have similar origin. 



Enamel of the Teeth. The minute structure of dental 

 tissue will be described hereafter, but as the enamel is 

 generally considered to be of epithelial origin, some ac- 

 count of it belongs here. 



O 



The edge of the jaw is first marked by a slight groove, 

 known as the dental groove, and is covered with a thick 

 ridge of epithelium, called the dental ridge (Plate XIX, 

 Fig. 144, 1 a, 2 a). The epithelium grows down in a 

 process ' which has been called the enamel germ (1 d). 

 This becomes doubled by the upward growth of the 

 dental germ (2, 3,/), which originates from connective 

 tissue. The epithelial cells become transformed into 

 enamel columns or prisms. 



3. CONNECTIVE SUBSTANCES OR TISSUES. 



The term connective tissue has been given to a variety 

 of structures which probably start from the same rudi- 

 ments, and have a near connection with each other. It 

 is unfortunate that a name descriptive of function should 

 be applied to structure, yet the present state of histology 

 requires an account of substances thus called. 



Connective tissues are all those which may be regarded 



