792 THE SKIN AND ITS SECRETIONS. 



Russo on acid cleavage it yields 25 per cent acetic acid and 60 per cent 

 glucosamine. 



In a dry state chitin forms a white, brittle mass retaining the form 

 of the original tissue. It is insoluble in boiling water, alcohol, ether, 

 acetic acid, dilute mineral acids, and dilute alkalies. It is soluble in 

 concentrated acids. It is dissolved without decomposing in cold con- 

 centrated hydrochloric acid, but is decomposed by boiling hydrochloric 

 acid. According to KRAWKOW the various chit ins behave differently 

 with iodine or with sulphuric acid and iodine, in that some are colored 

 reddish brown, blue, or violet, while others are not colored at all. 



Chitin may be easily prepared from the wings of insects or from the 

 shells of the lobster or the crab, the last-mentioned having first been 

 extracted by an acid so as to remove the lime salts. The wings or shells 

 are boiled with caustic alkali until they are white, afterward washed 

 with water, then with dilute acid and water. The pigments remaining 

 can be destroyed by permanganate. The excess of this last can be removed 

 by a dilute solution of bisulphite, washed with water and then extracted 

 with alcohol and ether. 



Hyalin is the chief organic constituent of the walls of hydatid cysts. From a 

 chemical point of view it stands close to chitin, or between it and protein. In 

 old and more transparent sacs it is tolerably free from mineral bodies, but in 

 younger sacs it contains a great quantity (16 per cent) of lime salts (carbonate, 

 phosphate, and sulphate). 



According to Lt JCKE l its composition is : 



C H N O 



From old cysts 45.3 6.5 5.2 43.0 



From young cysts 44.1 6.7 4.5 44.7 



It differs from keratin on the one hand and from proteins on the other by the 

 absence of sulphur, also by its yielding, when boiled with dilute sulphuric acid, a 

 variety of sugar in large quantities (50 per cent), which is reducing, fermentable, 

 and dextrogyrate. It differs from chitin by the property of being gradually 

 dissolved by caustic potash or soda, or by dilute acids; also by its solubility on 

 heating with water to 150 C. 



The coloring matters of the skin and horn-formations are of different 

 kinds, but have not been extensively studied. Those occurring in the 

 stratum Malpighii of the skin, especially of the negro, and the black 

 or brown pigment occurring in the hair, belong to the group of those 

 substances which have received the name melanins. 



Melanins. This group includes several different varieties of amorphous 

 black or brown pigments which are insoluble in water, alcohol, ether, 

 chloroform, and dilute acids, and which occur in the skin, hair, epithelium- 

 cells of the retina, in sepia, in certain pathological formations, and in the 

 blood and urine in disease. Of these pigments there are a few, such as 



1 Virchow's Arch., 19. 



