442 CHEMISTRY OF THE TISSUES. 



CHAPTER XXXVI. 



CHEMISTRY OF THE TISSUES. 



42. Epithelial Tissues. The epithelial tissues nails, 

 hair, epidermis, and epithelium, as well as horns and feathers 

 contain a small quantity of fat, and a substance which con- 

 stitutes the chief part of their bulk, and to which their form is 

 due. To this substance the name of keratin has been given. 

 It is prepared by removing the fat, etc., from any of the epi- 

 dermal tissues by boiling with ether, alcohol, water, and dilute 

 :u-id. As the elementary analyses of it do not agree, it is quite 

 possible that it is a mixture of several substances, but this is 

 not yet certainly made out. It is nearly allied to albumin, as 

 is shown by its yielding the same products, leucine and tyro- 

 sine, when decomposed by boiling with dilute sulphuric acid 



:>5). It contains sulphur, which seems to be in a very 

 loose state of combination. Hair, as is well known, becomes 

 blackened by lead sulphide when a leaden comb is used. To 

 show the presence of sulphur, put a few parings of nails into 

 a test-tube; add a little liquor potassae, and boil. Add a little 

 hydrochloric or sulphuric acid to the solution thus obtained. 

 Hydrogen sulphide will be given oil', and maybe recognized by 

 the smell. 



43. Connective Tissue. In the group of tissues so desig- 

 nated, there are several which do not seem very like one another. 

 Such arc mucous tissue, rcticular and ordinary connective tis- 

 sue, adipose tissue, cartilage, bone, and dentine. Their close 

 relation to one another is shown by their being linked together 

 by Intermediate forms, by one tissue sometimes passing into 

 another so that the boundary between them cannot be defined, 

 and by one occasionally replacing another. They all contain 

 substances which are either derived from albumin or are nearty 

 connected with it, and have received the name of albuminoids. 



44. Albuminoid^, These are nitrogenous, and resemble 

 albuminous bodies in composition, but differ from them in their 

 behavior with acetic acid, potassium ferrocyanide, nitric and 

 hydrochloric acids. They are mnein, gelatin, and chondrin. 



* 45. Mucin. This is found in fcetal connective tissue, 

 and although not present in the fasciculi is an important con- 

 stituent of tendon tissue. It occurs also in all mucous secre- 

 tions, and gives them a tenacious character. It is distinguished! 

 by its solutions not being coagulated or rendered turbid by 



