68 VETERINARY STATE BOARD 



to the extremity of the allantoid portion, where they end in an 

 expansion of placental ramifications. 



Describe the ductus arteriosus and the foramen ovale in the foetus. 

 Give their uses and state what remnants of these can be 

 found in mature life. 



The ductus arteriosus is a short vessel which connects the pul- 

 monary artery^ near its origin, to the posterior aorta. In fetal life, 

 the lungs do not functionate, hence the blood is not carried to them 

 but takes this short course to the aorta. This duct remains, but in 

 mature life is represented by a yellow elastic fibrous cord (the liga- 

 mentum arteriosum) . 



The foramen ovale is the opening between the right and left 

 auricles, through which the blood passes in fetal life. It is pro- 

 vided with a valve, the Eustachian valve (absent in horse and pig) , 

 that stretches from the mouth of the posterior vena cava to the 

 annulus, or thickened border of the foramen. The function of this 

 valve is to direct the blood-stream into the left auricle, from whence 

 it goes into the systemic circulation. In mature life, the foramen 

 ovale is represented by a depression of a thin membrane, surrounded 

 by the ring of Vieussens, or annulus ovalis. Occasionally the open- 

 ing persists throughout adult life. 



Describe the anterior vena cava. 



A large trunk extending from the entrance of the chest to the 

 right auricle. It is formed by the union of the two axillary and 

 the two jugular veins. It receives in its course: the internal 

 thoracic, vertebral, superior cervical and dorsal veins, and the 

 great vena azygos. 



Give the origin of the vena azygos and name its tributaries. 



Originates at the first lumbar vertebra and extends forward to 

 the sixth dorsal, where it terminates in the anterior vena cava. Its 

 tributaries are the satellite veins of the first lumbar and all of the 

 aortic intercostal arteries, right and left. 



Describe the pulmonary veins. 



The pulmonary veins, four to eight in number, arise from the 

 substance of the lung, emerging immediately above the origin of the 

 bronchi. They terminate in the left auricle, which they enter by way 

 of four to eight orifices. They have no valves. Their function is to 

 carry the blood from the lungs, where it has been purified, to the 

 left auricle to begin the systemic circulation. 



