Z PROTEINS 351 



hard and resistant layer. It consists of parallel prisms of hydroxyl- 

 apatite orientated perpendicular to the enamel surface. Its optics have 

 been investigated by W. J. Schmidt (1923, 1936/37). In a very young 

 state this layer is optically positive with respect to the axis of the 

 prisms, whereas in full-grown teeth the enamel assumes an optically 

 negative character. This change is explained by the following facts: 

 In the embryonic state the enamel prisms represent a Wiener body 

 with submicroscopic spaces between submicroscopic crystallites of 

 hydroxyl-apatite; hence, its positive double refraction is caused by 

 'marked rodlet birefringence. Later, when the enamel hardens, the 

 submicroscopic spaces are filled with material of a refractive power 

 similar to that of the crystallites ; consequently the form birefringence 

 disappears and the optically negative intrinsic birefringence of the 

 hydroxyl-apatite becomes visible. This behaviour proves that the 

 optical axis of the submicroscopic crystallites must run parallel to the 

 axis of the microscopically visible prisms. 



It had been assumed that the filling material would be exclusively 

 inorganic, but the electron microscope has shown that it is not. Scott 

 and Wyckoff (1946/47) have developed a method for preparing thin 

 replicas of pofished and slightly etched tooth sections. In order to 

 obtain undamaged replicas they must be freed by dissolving the section 

 in 18 % HCl and 2 % pepsin, whereupon the replica is shadowed. 



In such preparations the microscopic enamel prisms appear to be 

 surrounded by a thin organic sheath and inside the prisms there is 

 a very fine organic matrix (Frank, 1950). Enamel is not therefore 

 simply an inorganic coat of the tooth, but contains an organic frame 

 as well. This explains why even completely intact teeth are subject 

 to decay. 



Elastic tissue. Elastic tissue as found in the back of the neck (liga- 

 mentum nuchae) of the vertebrates differs from the connective tissue 

 of collagen in several characteristic properties. Being highly elastic 

 and resistant to tryptic digestion, its protein has been given a special 

 name, viz., "elastin". 



Threads of elastin have little birefringence, but their double 

 refraction can be enhanced by stretching (Schmidt, 1924). It was 

 therefore thought that elastin w^ould behave like rubber, with dis- 

 ordered chain molecules in the relaxed state and parallel molecules in 

 the stretched state (cf. Gross, 1949). 



