20 HUMAN PHYSIOLOGY. 



juices, out of the proteins of the food, a series of new proteins, knows as 

 proteoses and peptones. The chemic properties of these substances will be 

 considered in connection with the process of digestion. 



CONJUGATED OR COMBINED PROTEINS. 



The different members of this group are capable of being decomposed 

 by chemic methods into a protein and a non-protein substance; e. g., a color- 

 ing matter, a carbohydrate, or a nuclein. The chemic character of the non- 

 protein substance furnishes the basis for the following classification: 



CHROMO-PROTEINS. 



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

 corpuscles, of which it constitutes about 30 per cent, of the total weight. 

 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 oxyhemo- 

 globin, and in the venous blood as dioxy- or reduced hemoglobin. 

 When hydrolyzed by acids or alkalies, hemoglobin undergoes a 

 cleavage into a protein, globin, and a pigment hematin. 



(6) Myohematin. Myohematin is a protein supposed to be present 

 in muscle. It has never been isolated, hence its chemic features are un- 

 known. Spectroscopic examination indicates that it is capable of ab- 

 sorbing and again yielding up oxygen. For this reason it is believed 

 to be a derivative of hemoglobin. 



GLUCO-PROTEINS. 



(a) Mucin. Mucin is the protein which gives the mucus secreted 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 proteid and a carbohydrate termed mucose, 

 which is capable of reducing Fehling's solution. 



(6) Mucoids. The mucoids resemble the mucins though differing 

 from them in solubility and in not being precipitable from alkaline 

 solutions by acetic acid. They are found in the vitreous humor, 

 white of egg, cartilage, and in other situations. They differ slightly 

 one from the other in properties and chemic composition. They 

 yield on decomposition a carbohydrate. 



