THE PROTEINS ,,, 7 



pared, they are all distinguished by the very large amount of sulphur presei 

 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 extracted 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 framework of the 

 central nervous system, must be grouped by its general behaviour as well as 

 by 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 nuchae, 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 xanthoproteic and Millon's tests. 



Other members of this group sue fibroin, which forms the main substance 

 of silk, spongin, the horny framework of sponges, conchiolin, the ground 

 substance of shells, and perhaps the amyloid substance or lardacein whic] 

 have already mentioned in connection with the mucoids.' All these s 

 proteins present considerable differences in their qualitative and quantitatr 

 composition in amino-acids. Their proximate composition is shown n 

 Table given above (Abderhalden). 



We have finally to mention a miscellaneous collection of bodie 

 allied to the proteins and are distinguished by their extreme mso 



