REPRODUCTION. 667 



directly through a duct, the ductus arteriosus, which enters at a point 

 below the origin of the left carotid and subclavian arteries. A com- 

 parison of the blood distributed to the head and upper extremities, with 

 that distributed to the lower extremities, will show a larger percentage 

 of nutritive material and oxygen in the former than in the latter, 

 a fact which has been offered as an explanation of the more rapid 

 growth of the upper half of the body. As the blood passes through 

 the aorta, a portion is directed from the main current by the hypogas- 

 tric and umbilical arteries to the placenta, where it loses carbon 

 dioxid and gains oxygen, and changes in color from a bluish red to a 

 scarlet red. 



Parturition. At the end of gestation approximately 280 days 

 from the time of conception a series of changes occur in the uterine 

 structures which lead to an expulsion of the child, the placenta, and 

 decidua vera. To this process in its entirety the term parturition 

 is given. At this time, from causes not clearly defined, the uterine 

 walls begin to exhibit throughout their extent a series of slight con- 

 tractions which are somewhat peristaltic in character; these con- 

 tractions, which gradually increase in frequency and vigor, bring 

 about a dilatation of the internal os and a descent of the membranes 

 into the cervical canal. The pressure exerted by these membranes 

 during the time of the contraction materially assists in the relaxation 

 of the circular fibers and a dilatation of the external os. When the 

 dilatation has so far advanced that the diameter of the external os 

 attains a measure of 7 or 8 cm., the tension of the membranes becomes 

 sufficiently great to lead to their rupture and to a partial escape of 

 the amniotic fluid. With this event, the presenting part of the 

 child, usually the head, descends into the cervical canal. After a 

 short period of rest the uterine contractions return and rapidly 

 increase in vigor and duration. As a result of the pressure thus 

 exerted from all sides on the body of the child, the head gradually 

 descends into the vagina and finally emerges through the vulva to 

 be followed in a short time by the expulsion of the trunk and limbs, 

 and a discharge of the remaining amniotic fluid. With the expulsion 

 of the child the uterine contractions cease for a period of ten or 

 fifteen minutes, when they again recur, with the result of detaching 

 the placenta and expelling it into the vagina. It is then removed 

 by the co-operative action of the abdominal and perineal muscles. 

 The hemorrhage which would naturally occur with the detachment of 

 the placenta and the laceration of the maternal vessels is prevented 

 by the firm continuous contraction of the uterine walls, by which 

 the vessels are compressed and permanently closed. 



The Establishment of Inspiration and the Adult Circulation. 

 After the birth of the child and the detachment of the placenta, 

 there speedily occurs a decrease in the quantity of oxygen and an 



