THE PROTEINS 



119 



The keratins are produced by the modification of epithelial cells and 

 form the horny layer of the skin as well as the main substance of hairs, 

 wool, nails, hoofs, horns, and feathers. They are distinguished by their 

 insolubility in water, dilute acids or alkalies, and in the higher animals 

 pass through the alimentary canal unchanged. Although differing in 

 their elementary composition, according to the tissue from which they 

 are prepared, they are all distinguished by the very large amount of 

 sulphur present in their molecule. The greater part of this sulphur is 

 in the form of cystine, of which as much as 10 per cent, can be ex- 

 tracted from keratin. They also yield, on acid hydrolysis, tyrosine in 

 larger quantities than is the case with the ordinary proteins. 



Neurokeratin, which forms the basis of the neuroglial frame- 

 work of the central nervous system, must be grouped by its general 

 behaviour as well as bv its origin with the keratins. It resembles the 







other members of this class in its insolubility and in its high content in 

 sulphur. It is extracted from nervous tissues by boiling these with 

 alcohol and ether and then submitting the tissue to prolonged tryptic 

 digestion, which leaves the neurokeratin unaffected. 



Elastin is a constant constituent of the connective tissues, where 

 it forms the elastic fibres. In some localities, as in the ligamentum 

 nuchse, practically the whole tissue is made up of these fibres. Elastin 

 is insoluble in water, alcohol, or ether, or in dilute acids and alkalies. 

 It is slowly dissolved on prolonged treatment with gastric juice, but is 

 practically unaffected in the alimentary canal. It gives the xantho- 

 proteic and Millon's tests. 



