CHEMICAL BASIS OF THE ANIMAL BODY. 1197 



the proteids. Their percentage composition approaches that 

 of the proteids, and like these they yield, under hydrolytic 

 treatment, large quantities of leucine and in some cases tyrosine 

 and other characteristic products. 



Mucins. 



These are the substances which give to many animal secre- 

 tions, such as saliva, bile, synovial fluid, etc., their character- 

 istic ropy consistency. They may also be obtained by the use 

 of appropriate solvents from the tissues themselves, such as 

 sub-maxillary gland, tendons and umbilical cord. The general 

 phenomena of the formation of mucin by mucous cells, and more 

 particularly the characteristic behaviour of the mucous gran- 

 ules in relation to the secretory activity of the sub-maxillary 

 gland, leave but little doubt that mucin is to be regarded as 

 derived from the true proteids ; in conformity with this it yields 

 many of the reactions characteristic of the proteids (Millon's 

 and xanthoproteic), and by the action of mineral acids some 

 form of acid-albumin is usually obtained. During this treat- 

 ment (or with alkalis) moreover a second product general! v 

 makes its appearance, which belongs to the group of carbohy- 

 drates, is known by the name of animal gum and by heating 

 with acids may be made to yield a reducing sugar. Notwith- 

 standing the views which have frequently been advanced that 

 mucin is in reality a mixture of proteid and carbohydrate 

 material, it is now known with considerable certainty that it 

 is a unitary substance which, from what has been already said, 

 may be regarded as an animal glucoside to which the general 

 name of glycoproteid may be given. It further appears that the 

 substance at first secreted by the mucous cells may not be typical 

 mucin but a sort of mucinogen which readily gives rise to mucin 

 on treatment with dilute (-01 p.c.) caustic potash. If it be 

 assumed for the moment that there is only one kind of mucin, 

 then the following general statements as to this substance may 

 be additionally made. It is precipitated from its solutions by 

 acetic or hydrochloric acids, the precipitate being soluble in 

 excess of the latter but not of the former acid. In its precipi- 

 tated form it swells up strongly in water but does not go into 

 true solution; the addition of dilute alkalis (*1 — -2 p.c.) or of 

 lime-water leads to its ready solution, from which it can again 

 be precipitated by the addition of an acid. It may be extracted 

 from any mucigenous tissue by the use of dilute alkalis or 

 lime-water, and in solution is somewhat characteristically pre- 

 cipitated by basic lead acetate. Our knowledge of mucin is 

 however in an extremely transitional condition, and recent 

 investigations have shewn that probably the mucins derived 

 from different sources are really distinct substances, just as we 



