738 TEXT-BOOK OF PHYSIOLOGY 



blood-stream which at the same time receives carbon dioxid and perhaps 

 other waste products from the fetal blood-stream. The placenta thl 

 serves as a digestion and respiratory organ. The blood having undergo^ 

 these changes now leaves the placenta and returns to the fetus by the UT& 

 bilical vein. This blood is relatively rich in nutritive material and off 

 scarlet red color by reason of the presence of an increased amount of oxygeJ 

 As it passes into the abdominal cavity a portion, about one-half, of the bk><3 

 is directed by the ductus venosus into the inferior vena cava, while the ri 

 mainder is emptied into the portal vein, by which it is distributed to tM 

 liver and from which it emerges by the hepatic veins and is poured into thl 

 inferior vena cava. The blood in the vena cava is thus a mixture of venou] 

 blood from the lower extremities and liver, and oxygenated blood from thl 

 placenta. After its discharge into the right auricle the blood is directed bjj 

 a fold of the lining membrane, the Eustachian valve, through an opening id 

 the interauricular septum, the foramen ovale, into the left auricle. It theiS 

 flows through the auriculo-ventricular opening into the left ventricle, thend 

 into the aorta, and by its branches is distributed to all parts of the body! 



The blood from the head and upper extremities is emptied by the superioi 

 vena cava into the right auricle, but as it passes in front of the Eustachiafl 

 valve, it flows directly into the right ventricle and then into the pulmonarj 

 artery. On account of the unexpanded condition of the lungs and the 

 almost impervious condition of the pulmonary capillaries, but a small 

 portion^ of the blood passes through them, while the larger portion by fai 

 passes into the aorta directly through a duct, the ductus arteriosus, which 

 enters at a point below the origin of the left carotid and subclavian arteries, 

 A^ comparison 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 

 liver and upper half of the body. As the blood passes through the aorta, 

 a portion is directed from the main current by the hypogastric 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 ttye 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 contractions which are somewhat peristaltic in char- 

 acter; these contractions, 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 

 vagina. ^ After a short period of rest the uterine contractions return and 

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



