THIRD TREATISE 



ON THE RESPIRATION OF 



THE FCETUS IN THE UTERUS 



AND IN THE EGG 



Since the necessity of breathing is so essential to the 

 sustaining of life that to be deprived of air is the same 

 as to be deprived of the common light and the vital 

 spirit, it will not be out of place to inquire here how 

 it happens that the foetus can live though imprisoned 

 in the straits of the womb and completely deprived 

 of the access of air. For it is not enough to say that 

 the blood of the child is brought during the period of 

 gestation through the foramen ovale and the ductus 

 arteriosus^ and circulates well enough without the 

 movement of the lungs, while the mass of the blood 

 takes its course, after birth, through the lungs, which 

 it cannot traverse without the help of respiration. 

 For respiration serves another purpose than trans- 

 mitting the blood through the lungs ; otherwise the 

 lungs would be altogether superfluous, since the blood 

 could have been carried round by another passage as 

 is done in the uterus. Nay, the blood can pass 

 through the lungs themselves without the aid of 

 respiration, as has been pointed out elsewhere. 

 Besides, if the foetus which has breathed air for some 



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