DEVELOPMENT. 



297 



In the foetus, respiration is effected through the placenta, and the 

 lungs are functionless. In correspondence with this the circulation 

 of the foetus differs from that of the animal after birth (fig. 680). From 

 the heart the blood is driven into the descending aorta, which sends 

 off behind two large vessels to the placenta (umbilical or allanloic 

 arteries). The blood, returning from the placenta in the allantoic 

 vein, passes in great part through a connecting vessel (ductus venosus 

 Arantii) into the 

 inferior vena cava, 

 and thence in part, 

 passes into the 

 right auricle, but 

 the greater part 

 flows, in conse- 

 quence of a special 

 arrangement of 

 valves, directly into 

 the left auricle 

 through an opening 

 in the interauricu- 

 lar septum, called 

 the foramen ovale. 

 The blood \which 

 reaches the v right 

 ventricle passes 

 through a vessel 

 (ductus arteriosus 

 JBotalli), connect- 

 ing the pulmonary 

 artery with the 

 aorta, directly into 

 the systemic cir- 

 culation, except a 

 small portion which 

 goes to the lungs. 



N 



FIG. 630. Diagram of the arrangement of the principal vessels 

 in a human foetus (after Huxley). H, Ventricle; V, Auricle ; 

 Ao, Aortic trunk ; Cc, common Carotid ; Ce, external Carotid ; 

 Ci, internal Carotid; 8, subclavian artery; 1,2,3,4,5, the 

 arterial arches the persistent left aortic arch is not visible ; 

 Aod, descending aorta; O, Omphalomeseraic (vitelline) 

 Artery ; <7, Omphalomeseraic (vitelline) Vein ; U, Umbilical 

 (allantoic) arteries with their placental ramifications (U") ; 

 U', Umbilical (allantoic) vein ; Vp, Portal vein ; Vc, Vena 

 cava inferior ; C, anterior cardinal vein ; D, Ductus venosus 

 Arantii; DC, Ductus Cuvieri; Az, Azygos Vein; P, Lungs; 

 L, Liver; 2V, Umbilical vesicle (yolk-sac); Dv, Vitelline duct 

 (Ductus omphalomeseraicus) ; Am, Amnion. 



From this condition of the circulation, it results 

 that all the arterial vessels, except the allantoic vein, contain mixed 

 blood. 



As remains of the first stage of the circulation before the develop- 

 ment of the placenta, the Omphalomeseraic vessels an artery and a 

 vein which belong to the umbilical vesicle, still persist. 



The duration of gestation depends on the size of the body and the 



