1!)4 



TOPOGRAPHICAL ANATOMY OF THE 



umbilical vein traverses the liver mixetl with the blood that has been 

 (fathered from the intestines, stomach, spleen and pancreas ; and some 

 is conveyed by the ductus venosus direct to the vena cava. In the 

 horse, the absence of a ductus venosus necessitates the passage through 

 the liver of all the blood of the umbilical vein. The origin and course 

 of the caudal vena cava is the same in the embryo as in the adult. Its 

 opening into the right atrium, however, is directly facing the foramen 

 ovale, an opening in the inter-atrial septum, and it appears probable 

 that the blood brought to the heart by the caudal vena cava passes 

 throueh the foramen into the left atrium. 



Truncus brachiocephalicus. 



Ductus arteriosus. 



Aorta. 



I\ip;ht alrium 

 A. pulmonalis. 



Right ventricle. 



- A luilmonary vein. 

 Left atrium. 



ventricle. 



Frr;. 85. — The fecial heart viewed from tlie left. Tlie ductus arteriosus ha.'* lieeii 

 ojiened, and the ari'ows indicate the direction in which the lilood Mow.«. 



Blood is collected from the head and neck, thoracic limbs and part 

 of the wall of the thorax of the embryo in the same manner as in the 

 adult, and consequently is carried to the right atrium by the cranial 

 vena cava. From the right atrium it is sent into the right ventricle, 

 and thence into the pulmonary artery. This artery furnishes a branch 

 to each lung as in the adult; but, the lungs being unexpanded and as 

 yet functionless, the greater part of the blood of the pulmonary artery 

 is short-circuited into the aorta through the ductus arteriosus. 



The blood of the caudal vena cava, gaining the left atrium by the 

 foramen ovale, is there mixed with the relatively small quantity of 

 blood brought from the lungs by the pulmonary veins. The mixed 



