148 THE FIEST BREATH. 



hanging ; the changes of pressure in the arteries and veins respectively 

 during a check on the respiration ; the vis a tergo of the veins ; the effects 

 of a ligature on those vessels ; the action of irrespirable gases when 

 breathed, and the opposite conditions when oxygen gas or protoxide of 

 nitrogen are used. 



Among the striking proofs of the truth of this doctrine, that the pri- 

 mary cause of the circulation is the aeration of the Tblood, I 

 would particularly direct attention to the effects which en- 

 sue in the moment of birth at the first breath. That intercommunication 

 between the two sides of the heart, established through the foramen ovale 

 and through the ductus arteriosus, is suddenly put an end to. But this 

 is not through any change in the mechanism of the heart itself, nor be- 

 cause of any interruption in the action of the placenta. It is solely be- 

 cause of the calling into operation of the principle we have been here en- 

 forcing. Through the contact of the cold air, or other causes which might 

 be assigned, the inspiratory muscles make their first contraction and dis- 

 tend the lungs. At that instant, the commencing arterialization produces 

 a pressure, in the manner I have explained, of the venous upon the now 

 arterialized blood in the vessels of the pulmonary cells. There is no 

 other possible issue to such an action than an instant drain upon the 

 heart. The pulmonary or less circulation sets in with full vigor. The 

 blood is not driven by the heart to the lungs, but drained by the lungs 

 from the heart. If it were the heart's action that occasioned this sudden 

 increase of force, because of the strain thrown upon it through the shut- 

 ting off of the influence of the placenta, it is inconceivable why the cur- 

 rent should not continue to move through the great avenues already open 

 to it from the right to the left auricle through the foramen ovale, and 

 from the right ventricle into the aorta through the ductus arteriosus. 

 The arrest of its motion through these channels distinctly establishes that 

 the seat of the new action is in the lungs, and the final closure of the 

 foramen and shriveling of the duct confirm the correctness of this con- 

 clusion. 



Though it does not strictly belong to the subject now under consid- 

 eration, I can not avoid impressing on the reader the suddenness of the 

 effect that thus ensues on the taking of the first breath. It is a crisis in 

 the history of development. Of these changes by crisis much more will 

 be said in the second book, and their important bearings on the theory 

 of physiology pointed out. It is enough for the present purpose to com- 

 mend to the attention of those naturalists who deny that physiological 

 crises ever occur, the facts which have been considered in the preceding 

 paragraph. 



A doctrine which accounts with simplicity for such a long list of mis- 

 cellaneous facts commends itself to our attention at once. There are, 



