CHEMICAL BASIS OF THE ANIMAL BODY. 1199 



hydrochloric acid and in its distinctly greater resistance to the 

 action of acids and alkalis. 



Mucin of the umbilical cord. May be prepared by the method 

 employed for the mucin of the sub-maxillary gland. It appears 

 to differ from the other mucins in containing more nitrogen 

 and a considerable amount of sulphur: it lies in fact somewhat 

 midway between the proteids and true mucins. 



By prolonged boiling with sulphuric acid mucins yield 

 leucine and tyrosine, but the products of their decomposition 

 have not been as yet fully studied. 



Analyses of the several mucins exhibit differences in per- 

 centage composition which lie within somewhat similar limits 

 to those already assigned (p. 1165) to the proteids. But a 

 comparison of these shews that the mucins contain slightly 

 less carbon, markedly less nitrogen and correspondingly more 

 oxygen than do the proteids. 



Mucin-like substances, to which the general name of mucoids or 

 mucinoids has been given, are found in ascitic fluids, the vitreous 

 humour, the cornea and in the white of egg. They all yield a reduc- 

 ing substance when boiled with mineral acids and otherwise resemble 

 the true mucins. 



Gelatin or Glutin. 1 



The ultimate fibrils of connective tissue and the organic 

 matter of which bones are largely composed consist of a sub- 

 stance named in the first case 'collagen,' in the second 4 ossein.' 

 The}?- are obtained either by digesting carefully cleansed ten- 

 dons with trypsin, which dissolves up all the tissue-elements 

 except the true collagenous (gelatin iferous) fibrils, or by ex- 

 tracting bones with dilute acids in the cold, by means of which 

 the inorganic salts are dissolved and the ossein remains as 

 a swollen elastic mass which retains the shape of the original 

 bone. As thus prepared they are insoluble in water, saline 

 solutions and either cold dilute acids or alkalis ; in the former 

 however (acids) they swell up to a transparent gelatinous mass. 

 When subjected to prolonged boiling with water, more espe- 

 cially under pressure as in a Papin's digester, they are gradu- 

 ally dissolved, and the solution now contains true gelatin into 

 which they have been converted by hydrolysis, and has acquired 

 the characteristic property of solidifying into a jelly on cooling. 

 The conversion of collagen into gelatin may be still more easily 

 effected by a shorter boiling in presence of dilute acids, but in 

 this case, unless the process be carefully regulated, the first- 

 formed gelatin is further hydrolyzed into what are often spoken 

 of as gelatin-peptones. Although insoluble in dilute acids 



1 Glutin must not be confounded with the vegetable proteid ' gluten.' 



