COURSE OP NOEMAL BIRTH 79 



physiological necessity, but that external stimuli (lowered 

 temperature, pressure) ordinarily are the first causes of respira- 

 tion ; iu fact, exert their influence before the disturbed placental 

 circulation creates a desire for oxygen by the organism." 

 Increase of carbon dioxide and want of oxygen may irritate the 

 respiratory centre, which also may occur by the influence of 

 air upon the skin. 



In consequence of the first respiration, many circulatory 

 changes take place in the young animal. 



The expansion of the lungs in consequence of the dilata- 

 tion of the thorax, the blood of the right heart is strongly 

 aspirated into the lungs through the pulmonary artery. Now 

 the blood current no longer runs from the right to the left side 

 by the oval foramen, but from the right auricle into the right 

 ventricle and from here by the pulmonary artery into the lungs, 

 and not by Botalli's duct into the posterior aorta. Botalli's 

 duct closes, and since no blood flows through it, the intima 

 folds upon itself and the lumen disappears. According to 

 Ellenberger, this lumen is often already closed after four to six 

 weeks. Since no blood flows from the pulmonary artery to the 

 posterior aorta, blood pressure in the latter is immediately 

 lessened after parturition. After the pulmonary circulation 

 has been established, blood pressure in the left auricle 

 increases; the valve of the oval foramen places itself over the 

 opening, and the foramen ovale is closed, this being assisted by 

 the flow of the blood toward the right ventricle. After one or 

 two years the valve of the oval foramen is united to the septum, 

 while it may be completely closed in a six months old calf 

 (Franck). 



The increased blood pressure in the left heart causes 

 hypertrophy of its walls. 



The umbilical arteries become obliterated and form the 

 round ligaments of the bladder. The umbilical vein is com- 

 pressed, its lumen disappears, to form later the round ligament 

 of the liver. 



The body temperature of the young immediately after birth 

 is a little higher than in the mother (Franck). 



