THE SKIN AND ITS SECRETIONS. 323 



chitin, found in the cuticle-formation of invertebrates, are of 

 interest. 



Tunicin, or animal cellulose, occurs, as above mentioned, in the tunicata. 

 According to BERTHELOT, tunicin differs from ordinary cellulose, being 

 colored yellow by iodine, and by its slower conversion into sugar on boiling 

 with acids. Otherwise it is similar to ordinary cellulose. The sugar 

 obtained by boiling tunicin with acids is grape sugar, according to FRANCHI- 

 MONT. 



Chitin is not found in vertebrates. The horny layer which covers the inner 

 side of the gizzard of birds, may perhaps consist of a substance related to chitin. 

 In invertebrates the chitin occurs in many classes of animals ; it cannot be 

 positively asserted that true, typical chitin is found elsewhere than in articu- 

 lated animals ; in these it forms the chief organic constituent of the shell, etc. 



According to SUNDWIK, the composition of chitin is probably CaoHiooN 8 O S 8 

 4- w(H 3 O), where n may vary from 1 to 4. On boiling with mineral acids it 

 decomposes and yields, as LEDDERHOSE has shown, glucosamine. According 

 to SUNDWIK, a glucose is also probably hereby produced. He also claims that 

 chitiu is an aniido derivative of a carbohydrate of the formula C 8 oHi oO 6 o. 



In the 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 concentrated hydrochloric acid, 

 but is decomposed by, boiling hydrochloric acid. When chitin is dissolved in 

 concentrated sulphuric acid and the solution dropped into boiling water and 

 then boiled, we obtain a, substance (glucosamine or glucose) which reduces 

 copper suboxide in alkaline solutions. 



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, and lastly 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 the 

 albumins. In old and more transparent sacks it is tolerably free from mineral 

 bodies, but in younger sacks it contains a great quantity (16#) of lime-salts 

 (carbonate, phosphate, and sulphate). 



According to LUCRE, 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 albumin on the other by the 

 absence of sulphur, also by its yielding a variety of sugar in large quantities 

 (50$), which is reducible, fermentable, and dextro-gyrate when boiled with 

 dilute sulphuric acid. 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 been but little studied. Those occurring 

 in the Malpighian layer of the skin, especially of the negro, and 



