x THE GLYCO-PROTEIDS : MUCINS 533 



the denaturalisation of the true albumins in being a permanent, irre- 

 versible one. Gly co-pro teids are pronounced acids, for they redden 

 litmus paper and are precipitable by stronger acids. They resemble 

 the nucleo-albumins in being uncoagulable by heat (which does not 

 exclude the possibility that they become denaturalised) and in being 

 acid in character, but they differ from the nucleo-albumins in possessing 

 no phosphorus and in containing a carbohydrate. Being acids, most 

 glyco-proteids are precipitated by acetic acid, but they are only 

 slightly soluble in an excess of this acid, while globulins, nucleo- 

 albumins, and nucleo-proteids are readily soluble in an excess of acetic 

 acid. Mineral acids also precipitate glyco-proteids, but the precipitate 

 dissolves more readily in an excess of such acids. In solutions of 

 alkalies, alkali-carbonates, and in ammonia, all glyco-proteids are 

 readily soluble, neutral and in some cases even acid salts being 

 formed. Excess of an alkali, however small, quickly denaturalises 

 and decomposes glyco-proteids. 



(a) The Mucins 



Mucins are found in most of the slimy fluids occurring in the 

 body, and they cause the sliminess. Even when greatly diluted they 

 form more or less adhesive solutions, which may be pulled out into 

 viscous threads. They are excreted normally, partly by the goblet 

 cells found on the surface of all mucous membranes, such as the 

 respiratory and alimentary systems, the bile-ducts, urinary passages, 

 etc., and partly by deeply situated mucous glands, and in particular by 

 the submaxillary gland. Mucins are also found amongst invertebrates 

 for example, in snails, the skin of which is covered with mucin. 

 Other bodies closely related to the mucins and forming the transition 

 to the mucoids are found in connective tissues for example, in tendons, 

 the vitreous humour, the umbilical cord, etc., and will be discussed 

 under the heading of the mucoids. In some animals the mucins are 

 replaced by nucleo-proteids, which latter also possess the same slimy 

 character. 



The mucin of the submaxillary gland of the ox, apart from older 

 investigations, has been studied specially by Obolensky l and Land- 

 wehr, 2 by Hammarsten 3 and his pupil Folin ; 4 the mucin of the 

 respiratory tract by Friedrich Muller ; 5 that of the bile by Landwehr, 2 



1 Obolensky, Hoppe-Seylers Med.-chem. Untersuch. p, 590 (1871). 



2 H. A. Landwehr, Zeits. f. phys. Chem. 5. 371 (1881) ;'6. 74 (1881) ; 9. 361 (1885). 

 a O. Hammarsten, ibid. 12. 163 (1887). 4 0. Folin, ibid. 23. 347 (1897). 



5 Friedrich Muller, Zeitschr. /. Biol. 42. 468 (1901), (here will be found a review 

 of the older papers by himself and his pupils). 



