The keratinized tissues 



65 



tissues are to be sought in : (a) the hardness and coherence of the inter- 

 cellular contents, and (b) the intercellular adhesion. The properties listed 

 in Table 5 give grounds for saying that in hard keratins the fibrous intra- 

 cellular protein is more plentiful, harder and more completely fused into 



Table 5. Properties of " Hard " and " Soft " Keratins 



* This distinction is more apparent than real and is partly due to the 

 more massive character of the hard keratins. 



t The lipids of the softer keratins may act as " plasticizer." When 

 extracted the materials become very tough and hard. 



I This difference in the ratio of the basic amino acids was deduced 

 by Block on the basis of a large number of analyses. Block regards 

 the basic amino acids as forming a characteristic structural element in 

 proteins and he distinguishes sharply between the two classes of keratin 

 prefering to call the soft keratins " pseudo-keratins." Recent analysis 

 seems to show that the ratios are by no means exact integers. 



continuous masses and that the adhesion between the cells is more complete 

 and more persistent. In the soft keratins the fibrils may be less completely 

 or less strongly fused. The fibrillar content of soft keratins partly derives 

 from keratohyalin (see p. 95); this substance appears deficient in the 

 covalent cross-linking responsible for cohesion (p. 234) and thus may more 

 readily break up. The thickness of the hardened layers (stratum lucidum 



