ELEMENTS OF COMPOSITION. 21 



The combination of haemoglobin with nitrous oxide (Hermann) conducts 

 itself in a manner similar to its combination with oxygen. 



C. Histogenic Derivatives of the Albuminous Substances 

 or Albuminoids. 



Keratin. Mucin. Colloid. 



We come now to certain matters which receive in general but little 

 attention. They are related to the protein compounds, and take their 

 origin apparently from the latter within the body. They also are colloids. 

 Their decomposition products are, in many respects, very similar to those 

 of the albuminous substances. 



In the older cells of horny tissue, of epithelium, of nails and hair, as 

 well as in the analogous structures of animals, there exists a mixture or 

 compound difficult to isolate in a pure state, and insoluble in water. It 

 may contain up to five per cent, of sulphur, and is partially soluble in 

 alkalies. Its. decomposition products, among which leucin and a large 

 quantity of tyrosin are found, indicate a close relationship with the pro- 

 tein substances. To this compound the name keratin has been given. 



Under the name of mucin is known a substance, sometimes gelatinous, 

 sometim'es dissolved, which occurs in the secretions of the mucous mem- 

 branes, in synovia and the vitreous humour of the eye, in the gelatin of 

 WJiarton of the umbilical cord, in several connective-tissue structures, and 

 finally, in certain pathological products (mucous tissue). This substance 

 does not coagulate on being heated. It is thrown down in flakes by acetic 

 acid, and is not redissolved by an excess of the same. Alcohol produces 

 a species of stringy coagulum in solutions containing mucin, but this 

 dissolves again in warm water. In many other respects mucin resembles 

 the protein compounds ; its reaction with sugar and sulphuric acid is also 

 the same. It appears to contain no sulphur, but is, on the other hand, 

 rich in phosphate of lime (Scherer). Mucin, which is not diffusible, 

 manifests fermenting properties. It appears to form a kind of peptone 

 (Eichwald). 



Colloid matter may also be mentioned here : a usually homogenous 

 substance of some consistence, insoluble in acetic acid, but not, like 

 mucin, precipitated by the latter. It is soluble, on the other hand, in 

 alkalies. It is generally met with as a pathological product of the 

 transformation of tissues (colloid degeneration), but also normally at cer- 

 tain periods of life, particularly in the thyroid gland of man. 



15. 

 Substances yielding Glutin. 



From experience, we know that the important group of glut in-yield- 

 ing materials takes its origin from protein compounds. These principles 

 only occur in animal organisms, and constitute a large part of our body, 

 in the form of interstitial matter in structures composed of connective 

 tissue, of bone, and of cartilage. We understand by glutin-yielding 

 materials, compounds containing nitrogen and sulphur, completely in- 

 soluble in cold water, but which may be gendered soluble by prolonged 



