CHAPTER XVIII 



EPITHELIAL AND CONNECTIVE TISSUES 



EPITHELIAL TISSUE (KERATIN) 



THE albuminoid keratin constitutes the major portion of hair, horn, 

 hoof, feathers, nails, and the epidermal layer of the skin. There is a 

 group of keratins the members of which possess very similar properties. 

 The keratins as a group are insoluble in the usual protein solvents and 

 are not acted upon by the gastric or pancreatic juices. They all re- 

 spond to the xanthoproteic and Millon reactions and are characterized 

 by containing large amounts of sulphur. Keratin from any of its 

 sources may be prepared in a pure form by treatment, in sequence, with 

 artificial gastric juice, artificial pancreatic juice, boiling alcohol, and 

 boiling ether, from twenty-four to forty-eight hours being devoted 

 to each process. 



The percentage composition of some typical keratins is given in the 

 following table: 



The composition of human hair is influenced by its color and by the 

 race, sex, age and purity of breeding of the individual. 3 It may be dif- 



1 Mulder: Versuch einer allgem. physiol. Chem., -Braunschweig, 1844-51. 



2 Horbaczewski: Ladenburg's Handworterbuch d. Chem., 3. 

 8 Rutherford and Hawk: Jour. Biol. Chem., 3, 459, 1907. 



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