CHANGES AT BIRTH. 803 



limbs. The larger portion of the blood, however, which passes into the descend- 

 ing aorta, is sent out of the body to the placenta. This duty is principally per- 

 formed by the right ventricle, which after birth is charged with an office some- 

 what analogous, in having to propel tlie blood through the lungs. But the 

 passage of the blood through the vessels of the umbilical cord and placenta is 

 longer and subject to greater resistance than that of the pulmonary chculation, 

 and the right ventricle of the foetus, although probably aided by the left in the 

 placental circulation, also takes a large shave in the systemic through the lower 

 half of the body ; and this, ijerhaps, may be the reason wliy the wall of the 

 right equals in thickness that of the left ventricle in the fa^tus. 



Sabatier was the first to call attention particularly to the action of the Eusta- 

 chian valve in separating the currents of blood entering the right auricle by the 

 superior and inferior venaj cavse. (Traite d'Anat., vol. ii., p. 22i.) This sepa- 

 ration, as well as that occm-ring between the currents passing through the aortic 

 arch and the ductus arteriosus into the descending aorta, were illustrated experi- 

 mentally by John Reid. (See art. '• Heart," in Cyclop, of Anat. and Phj'siol., and 

 Edin. Med. and Sui-g. Journal, ISo.j.) A striking confirmation of the extent to 

 which the last mentioned division of the two currents of the foetal blood may 

 take place, without distm'bance of the chculation up to the time of bii-th, is 

 afforded by the examples of malformation in which a complete obliteration has 

 existed in the aortic trunk immediately before the place of the union of the 

 ductus arteriosus with the posterior part of the aortic arch. 



CHANGES IN THE CIRCULATION AT BIRTH. 



The changes which occur in the organs of circulation and respiration 

 at birth, and lead to the establishment of their permanent condition, are 

 more immediately determined by the inflation of the, lungs with air in 

 the first respiration, the consequent rapid dilatation of the pulmonary 

 l)lood-vessels with a greater quantity of blood, and the interruption to 

 the passage of blood through the placental circulation. These changes 

 are speedily accompanied by shrinking and obliteration of the ductus 

 arteriosus, in the space between the division of the right and left pul- 

 monary arteries and its junction with the aorta, and of the umbilical 

 arteries from the hypogastric trunk to the place of their issue from 

 the body by the umbilical cord ; — by the cessation of the passage of 

 blood through the foramen ovale, and somewhat later by the closure 

 of that foramen, and by the obliteration of the umbilical vein as 

 far as its entrauce into the liver, and of the ductus venosus within that 

 organ. 



The process of obliteration of the arteries appears to depend at first 

 mainly on the contraction of the coats, but this is very soon followed 

 by a considerable thickening of their substance, reducing ra^mlly their 

 internal passage to a minute tube, and leading in a short time to final 

 closure, even although the vessel may not present externally any con- 

 siderable diminution of its diameter. It commences at once, and is per- 

 ceptible after: a few respirations have occurred. It makes rapid pro- 

 gress upon the first and second days, and by the third or fourth days 

 the passage through the umbilical arteries is usually completely inter- 

 rupted. The ductus arteriosus is rarely found open after the eighth 

 or tenth day, and by three weeks it has in almost all instances become 

 completely impervious. 



The process of closure in the veins is slower, there not being the 



same thickening or contraction of their coats ; but they remain empty 



of blood and collapsed, and by the sixth or seventh day, are generally 



closed. 



3 F 2 



