HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 979 



connective tissue, surrounds all of these parts; it contains connective- 

 tissue fibrils, connective-tissue corpuscles and lymphoid cells, even 

 elastic fibers. The gelatinous substance contains mucin. Numerous 

 lymph-channels, lined with endothelium, traverse the jelly; lymph- 

 vessels and blood-vessels are absent. Nerves are found from 3 to 8 or 

 ii cm. from the navel. 



The jelly contains two forms of mucin, like that of the tendons, also globulin 

 (myosin?) and albumin. 



The fetal circulation that exists after the development of the allan- 

 tois pursues the following course: the blood of the fetus passes by way 

 of the two umbilical arteries (from the hypogastric) through the um- 

 bilical cord to the placenta, where the arteries break up into the capil- 

 laries of the placental villi. Returning from these the blood collects 

 in the umbilical vein (its color is scarcely a little brighter as compared 

 with that of the venous blood in the umbilical arteries). The um- 

 bilical vein (Fig. 387, 3, u^ turns upward from the navel and, passing 

 under the border of the liver, it anastomoses with the portal vein (a) 

 and continues as the ductus venosus of Arantius to the inferior vena 

 cava, which then conveys the blood to the right auricle. From here 

 the Eustachian valve and the tubercle of Lower (Fig. 384, 6, tL) deflect 

 most of the blood through the foramen ovale into the left auricle, from 

 which, on account of the presence of the valve of the foramen ovale, it 

 cannot flow back into the right auricle. From the left auricle the blood 

 passes through the left ventricle, the aorta and the hypogastric artery 

 back into the umbilical arteries. The blood of the superior vena cava in the 

 fetus, by reason of its peculiar entrance, passes from the right auricle 

 into the right ventricle (Fig. 384, 6, Cs). From here it enters the 

 pulmonary artery (Fig. 384, 7, p), which conveys it into the aorta 

 through its prolongation, the ductus arteriosus Botalli (J5), which 

 empties into the aortic arch. Only a little blood passes by way of the 

 small branches of the pulmonary artery (1,2) through the lungs. The 

 course of the blood makes it clear that the head and the upper extremities 

 are supplied with purer blood than is the remainder of the trunk, which 

 also receives an admixture of the blood from the superior vena cava. 

 After birth the umbilical arteries are obliterated, and become the 

 lateral ligaments of the bladder; their lower portion, however, persists 

 as the superior arteries of the bladder. The umbilical vein also is 

 obliterated and becomes the round ligament; and likewise the ductus 

 venosus of Arantius. Finally the oval foramen closes, and the ductus arter- 

 iosus Botalli becomes obliterated to form the ligamentum arteriosum. 



The relation of the fetal membranes in multiple pregnancies is as follows: 

 (i) In the presence of twins there are two entirely separate ova, with two placentas 

 and two reflex deciduas. (2) Two entirely separate ova have but one reflex 

 decidua, the placentas becoming adherent, while their vessels are separate. The 

 chorion is double, but not separable into two lamellae at its surface of contact. 

 (3) When there are one reflex decidua, one chorion, one placenta, two umbilical 

 cords, and two amnia, the vessels anastomose in the placenta, and, therefore, 

 the central stump of the umbilical cord of the first born of twins should always 

 be tied. Under such circumstances there has been either one ovum, with a double 

 yolk, or two germinal vesicles in one yolk; or it must be assumed that two sepa- 

 rate ova have subsequently united, with absorption of the contiguous parts of 

 the chorion. (4) When the conditions just described are present, except that 

 there is but one amnion, they are due to the formation of two embryos in the 

 same germinal area of the same germinal vesicle. 



