22 HUMAN PHYSIOLOGY. 



gastric and pancreatic juices, out of the proteids of the food, a series 

 of new proteids,, knows as proteoses and peptones. The chemic prop- 

 erties of these substances will be considered in connection with the 

 process of digestion. 



CONJUGATED OR COMBINED PROTEIDS. 



The different members of this group are capable of being de- 

 composed by chemic methods into a proteid and a non-proteid sub- 

 stance ; e, g., a coloring matter, a carbohydrate, or a nuclein. The 

 chemic character of the non-proteid substance furnishes the basis 

 for the following classification : 



1. CHROMO-PROTEIDS. 



(a) Hemoglobin. Hemoglobin is the coloring matter of the red 

 corpuscles, of which it constitutes about 94 per cent. It 

 possesses the power of absorbing oxygen as it passes through 

 the lung capillaries and of yielding it up to the tissues as it 

 passes through the tissue capillaries. In the arterial blood 

 it is known as oxyhemoglobin, and in the venous blood as 

 dioxy- or reduced hemoglobin. When hydrolyzed by acids 

 or alkalies, hemoglobin undergoes a cleavage into a proteid, 

 globin, and a pigment hematin. 



(fr) Myohematin. Myohematin is a proteid supposed to be 

 present in muscle. It has never been isolated, hence its 

 chemic features are unknown. Spectroscopic examination in- 

 dicates that it is capable of absorbing and again yielding 

 up oxygen. For this reason it is believed to be a derivative 

 of hemoglobin. 



2. GLUCO-PROTEIDS. 



(a) Mucin. Mucin is the proteid which gives the mucus secretecl 

 by the epithelial cells of the mucous membranes and related 

 glands its viscid, tenacious character. It is also a constituent 

 of the intercellular substances of the connective tissues. It 

 is readily precipitated by acetic acid. When heated with 

 dilute acids, mucin undergoes a cleavage into a similar pro- 

 teid and a carbohydrate termed mucose, which is capable 

 of reducing Fehling's solution. 



