178 PKACTICAL PHYSIOLOGY 



is the most important, the resulting coagula including fibrin (in clotted 

 blood),. casein (in clotted milk), myosin (in ' rigor mortis' muscle). These 

 bodies will be studied in their proper places. 



IV. Compound Proteids. These are combinations of native proteids 

 with another organic substance. 1 This latter may be : 



(a) a carbohydrate glucoproteids. 



(b) nucleic acid nucleins. 



(c) an iron containing pigment haemoglobin. 



(d) Further compounds also exist where two different kinds of 

 proteid are combined, such as nucleo-proteids (nuclein + native proteid) 

 and histones (albumin .+ protamin). To the latter group belongs 

 globin, the proteid which is separated from haemoglobin by decom- 

 posing it with acids or alkalies (see p. 193). 



It will only be possible to merely indicate the chief properties of 

 these bodies here, a more complete study of them being reserved 

 for the advanced course. 



(a) G-luco-proteids. The most important member of this group is 

 Mucin. 



EXPERIMENT XVI. Collect some saliva in a test tube, add to it a 

 drop of 10 per cent, acetic acid ; a stringy precipitate of mucin results. 

 Add a few drops of weak sodium carbonate solution when the pre- 

 cipitate will redissolve. 



EXPERIMENT XVII. Mucin has been prepared from connective 

 tissue where it is very abundant, by extracting the latter with a 

 weak alkali (lime water). The mucin has been precipitated by a 

 weak acid. The resulting precipitate has then been boiled for about 

 ten minutes with hydrochloric acid (1 part concentrated acid + 3 parts 

 water), and the resulting solution cooled and neutralised. Examine 

 portions of the resulting solutions. Divide the solution into two 

 portions, a and b. 



To (a) apply the biuret reaction a violet or pink colour is pro- 

 duced, showing the presence of the proteid moiety. 



To (b) add a drop of copper sulphate solution and, if necessary, some 

 caustic alkali till a blue solution is obtained. Now boil, when 

 reduction to cuprous oxide will occur, demonstrating the presence of 

 the carbohydrate moiety. 



Besides forming the ground substance of the connective tissues, 

 mucin is also secreted on to the surface of all mucous membranes, 

 where it acts as a lubricant. 



*In contrast to album inates, which are compounds of native proteids with 

 inorganic substances. 



