QUESTIONS AND ANSWERS 67 



of the periostie network that enters the superficial Haversian canals. 

 In the flat and short bones, there are no arteries of the first order. 

 Describe the systemic circulation. 



The blood enters the left auricle from the limgs through the 

 pulmonary vein, passes through the bicuspid valve into the left 

 ventricle, through the semilunar valve into the common aorta, an- 

 terior and posterior aorta, from thence it is distributed to all parts 

 of the body, passing through arteries, capillaries and into the veins 

 back to the right auricle, through the anterior and posterior vena 

 cava. At the right auricle, the pulmonary circulation begins. 



Describe the pulmonary circulation. 



The blood reaches the right auricle through the anterior and pos- 

 terior vena cava and passes through the tricuspid valve into the right 

 ventricle, thence through the semilunar valve into the pulmonary 

 artery, through which it is carried to the lungs. In the lungs it goes 

 through a fine network of capillaries and returns through the pul- 

 monary vein to the left auricle, here to begin the systemic circu- 

 lation. 



Describe the fetal circulation of the blood. 



The blood, after interchanging gases in the placenta, enters the 

 umbilical vein and is carried to the liver. In the substance of the 

 liver, it is mixed with the venous blood from the intestines and 

 posterior parts, through the medium of the ductus venosus, and at 

 last arrives at the right auricle. From here it passes into the left 

 auricle through the foramen ovale, thence into the left ventricle and 

 aorta. The greater portion is driven into the vessels that supply 

 the head, neck and fore limbs, the remainder passes backward in the 

 posterior aorta. After the fluid has circulated in the anterior part 

 of the body, it is returned to the right auricle by the anterior vena 

 cava. From the right auricle, it passes to the right ventricle, and 

 from this cavity it is pumped into the pulmonary artery, theitee 

 through the ductus arteriosus into the posterior aorta, which carries 

 it to the hinder parts of the body. The veins of the hind parts unite 

 in forming the umbilical artery, which conveys the blood to the 

 placenta. The umbilical artery is practically a continuation of the 

 internal iliacs. (The ductus venosus only exists in ruminants.) 



Describe the umbilical arteries of the foetus. 



The umbilical arteries arise from the internal iliacs and pass 

 along the sides of the bladder. Escaping at the umbilicus, they 

 arrive at the terminal extremity of the amniotic portion of the 

 cord, and, giving off some branches to the amnion, they are continued 



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