USES OF THE PLACENTA 815 



centa are so closely united that they cannot be separated from each 

 other. In parturition the curling arteries and the veins on the uterine 

 surface of the placenta are torn off, and the placenta then consists of 

 the parts just described; the torn ends of the vessels attached to the 

 uterus are closed by the contractions of the surrounding muscular fibres ; 

 and the blood that is discharged is derived mainly from the placenta 

 itself. 



Uses of the Placenta. The placenta is the respiratory, excretory and 

 nutritive organ of the foetus. Its action as a respiratory organ has already 

 been mentioned in connection with the physiology of respiration. It cer- 

 tainly serves as an organ for the elimination of carbon dioxide, and 

 probably for other products of excretion. It is the only source of 

 materials for the development and nutrition of the foetus. It is thought 

 that cells derived from the serotina elaborate a liquid called uterine milk, 

 which is absorbed by the foetal tufts. This liquid has been collected 

 from between the foetal tufts of the placenta of the cow and has been 

 found to contain fatty matters, albuminous matters and certain salts, but 

 no sugar or casein. It is not probable, however, that such a liquid exists 

 in the human placenta; although "uterine milk" of the ruminants was 

 mentioned by Haller and has been alluded to by even earlier writers. 



