106 



PHYSIOLOGY 



the tissue to prolonged tryptic digestion, which leaves the neurokeratin 

 unaffected. 



Elastin is a constant constituent of the connective tissues, where it foims 

 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 xanthoproteic and Millon's tests. 



Other members of this group are 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 which we have already mentioned in 

 connection with the mucoids. All these sclero -proteins present considerable differ- 

 ences in their qualitative and quantitative composition in amino-acids. Their proxi- 

 matte composition is shown in the Table given above (Abderhalden). 



We have finally to mention a miscellaneous collection of bodies which are allied 

 to the proteins and are distinguished by their extreme insolubility. They are often 

 designated as albumoids. Of their composition we know practically nothing. Under 

 this name are grouped such substances as those forming the membrana propria of 

 glands, the sarcolemma of striated muscle, the albumoid of the crystalline lens, the 

 ground substance of the chorda dorsalis, the organic basis of fish scales, and many 

 similar Mil.staiiees. In every case the substance is characterised necessarily according 

 to its place of origin, little or nothing being kimun as to its chemical composition. 



