CH. IV.] THE CONNECTIVE TISSUES. 37 



There are probably several mucins, but they all agree in the 

 following points : 



(a) Physical character : viscid and tenacious. 



(b) Precipitability from solutions by acetic acid. They all 

 dissolve in dilute alkalis, like lime-water. 



(c) They are all compounds of proteid, with a carbohydrate 

 material which has been provisionally called animal gum ; 

 by treatment with dilute mineral acid this is hydrated into 

 a reducing but non-fermentable sugar-like substance. 



The substance mucin, when it is formed within cells (goblet 

 cells, cells of mucous glands), is preceded in the cells by granules 

 of a substance which is not mucin, but is readily changed into 

 mucin. This precursor, or mother-substance of mucin, is called 

 mucigen or rmtcinogen. 



Keratin, or horny material, is the substance found in the sur- 

 face layers of the epidermis, in hairs, nails, hoofs, and horns. It 

 is very insoluble, and chiefly differs from proteids in its high 

 percentage of sulphur. Keratin is a member of a heterogeneous 

 group of proteid-like substances which are called albuminoids, 

 and several more members of this group we shall have to consider 

 in our next chapter on the connective tissues. 



CHAPTER IV. 



THE CONNECTIVE TISSUES. 



THE connective tissues are the following : 



1. Areolar tissue. 



2. Fibrous tissue. 



3. Elastic tissue. 



4. Adipose tissue. 



5. Retiform and lymphoid tissues. 



6. Jelly-like tissue. 



7. Cartilage. 



8. Bone and dentine. 



9. Blood. 



At first sight these numerous tissues appear to form a very 

 heterogeneous group, including the most solid tissues of the body 

 (bone, dentine) and the most fluid (blood). 



