654 



MAMMALIA 



[CH. 



It is an interesting feature of the arterial system of a Mammal 

 that in consequence of the embryo receiving its oxygen from the 

 maternal blood, the connection between the systemic and pulmonary 

 arches persists in an unreduced condition until birth. This connec- 

 tion is known as the ductus arte- 

 riosus, and through it the blood from 

 the right ventricle is passed direct 

 to the dorsal aorta. The pulmonary 

 arteries are quite small during this 

 period, and by these arrangements 

 circulation of blood through the as 

 yet functionless lungs is avoided. 

 At birth the ductus arteriosus 

 shrinks and is rapidly reduced to 

 a solid cord, while the enlarging 

 pulmonary vessels provide for the 

 deviation of the venous blood to the 

 now expanded lungs (18, Fig. 323). 



In the venous system the blood 

 from the head is returned by ex- 

 ternal and internal jugular veins, 

 the former being much the larger. 

 The caudal vein is continued directly 

 into the inferior vena cava, so that 

 there is no longer even the outward 

 appearance of a renal portal system. 

 This is due to the formation of a 

 cross connection or anastomosis be- 

 tween the two posterior cardinal veins 

 immediately behind the kidneys and 

 to the subsequent disappearance of 

 the right posterior cardinal in front 

 of this, the left posterior cardinal 

 extending through the subcardinal 

 branch round the inner side of the 

 left kidney to join the inferior vena 

 cava where it issues from the kidneys. In consequence of this 

 re-arrangement the left posterior cardinal assumes the appearance 

 and functions of a posterior portion of the inferior vena cava. The 

 right posterior cardinal persists only as the right common iliac vein 

 into which the veins of the right leg empty themselves and as the 



13 



10 



13 



10 



FIG. 324. Diagram to show arrange- 

 ment of the principal veins in a 

 Mammal. 



1. Sinus venosus gradually dis- 

 appearing in the higher forms. 

 2. Ductus Cuvieri = superior vena 

 cava. 3. Internal jugular = an- 

 terior cardinal sinus. 4. Ex- 

 ternal jugular = sub-branchial. 

 5. Subclavian. 6. Posterior car- 

 dinal front part = venae azygos 

 andhemiazygos. 7. Inferiorvena 

 cava. 9. Caudal. 10. Sciatic 

 = internal iliac. 13. Femoral 

 = external iliac. 





