THE KERATINIZED TISSUES 79 



The " neurokeratin," a material remaining when nervous tissue is 

 exhaustively extracted, analysed by Block and found to contain cystine, 

 now appears to be simply a resistant product of decomposition. Histo- 

 chemists continue to describe a definite, cystine-containing " neurokeratin 

 network." 



Keratin-like materials in extracellular situations are found in the 

 linings of the gizzards of gallinaceous birds (see pp. 30 and 107) and in 

 egg-shell membranes. 



