100 PHYSIOLOGICAL CHEMISTRY 



is previously removed by adding an oxalate to the milk, the para- 

 casein is formed from caseinogen by rennin, but will not precip- 

 itate. Subsequent addition of excess of a soluble calcium salt 

 will cause precipitation even if the milk has been boiled to de- 

 stroy the rennin. In the transformation of caseinogen into para- 

 casein a protein like substance called albumose, or "whey pro- 

 tein" is split off. This clotting is the first step in the digestion 

 of casein in the stomach, and serves to prevent the milk from 

 passing 011 too quickly into the intestine. 



Vitellin. Vitellin is found in egg yolk and may be prepared 

 by extracting the yolk with ether to remove lecithin, cholesterol, 

 etc. The residue is dissolved in salt solution and reprecipitated 

 by pouring into a large volume of water. To remove the last of 

 the lecithin it is necessary to extract with boiling alcohol. 



Hemoglobins. This group often is called the group of chro- 

 moproteins, since its members usually are colored substances. 

 The most important of them is hemoglobin or oxyhemoglobin, 

 the red coloring matter of the blood. This compound is con- 

 tained in the red corpuscles in higher animals. In some of the 

 lower animals it is simply dissolved in the blood plasma. It, or a 

 substance closely allied to it, occurs also in red muscle. The hemo- 

 globin fulfils an important function in the body, that of trans- 

 porting oxygen from the lungs to the tissues. If hemoglobin is 

 exposed to a plentiful supply of oxygen, as is the case when the 

 red corpuscles are passing through the capillaries surrounding 

 the tiny air spaces, the alveolae in the lungs, it takes up oxygen 

 and is converted into oxyhemoglobin. When the corpuscles con- 

 taining this oxyhemoglobin reach the tissues, again they pass 

 through a fine network of capillaries. Here, however, the oxygen 

 supply is very low, as oxygen is rapidly used up by the cells in 

 oxidation processes. The oxyhemoglobin gives up its oxygen, 

 becoming hemoglobin again, and the oxygen passes out through 

 the capillary walls to the region of low oxygen supply. Re- 

 turning to the lungs, the hemoglobin takes up a fresh supply of 

 oxygen, and again carries it to the tissues. This property of 

 hemoglobin may be demonstrated easily as follows: On adding 



