992 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



I. 



(into which the ductus Botalli empties) becomes the permanent aorta, the right, 

 the right subclavian (5) . The uppermost arch on each side becomes the origin 

 of the carotids (Ci, Ce} . According to Zimmermann there occur in man and in rab- 

 bits the rudiments of a hitherto unknown transitory arch between the lowermost 

 and the next higher aortic arch on each side. 



The arteries of the first and the second circulation have already been con- 

 sidered. With the disappearance of the omphalomesenteric circulation, there 

 is but one omphalomesenteric artery present, and this soon gives off a branch 

 to the intestine. Later, the umbilical artery atrophies, so that the trunk of the 

 intestinal artery (the superior mesenteric artery, the largest of all arteries) is 

 originally an omphalomesenteric artery. 



The Veins of the Body. The veins that first develop in the body of the embryo 

 itself are the two cardinal veins: on each side an anterior (Fig. 386, I, cs) and 

 a posterior (ci} , which, passing toward the heart, unite on each side to form a 

 large trunk, the duct of Cuvier (D C}. The latter joins the venous portion of 

 the heart. The anterior cardinal veins give off the subclavian veins (b b) and the 

 common jugular veins, which divide into the internal (Ji) and external (le) 



jugular veins. In addition, 

 there is a transverse anas- 

 tomosing branch passing ob- 

 liquely from the left (where 

 it divides) toward the right, 

 and emptying into its trunk 

 at a somewhat lower level. 

 In the definitive development 

 (II) this anastomosing branch 

 (A s) becomes large (forming 

 the left innominate vein) ; 

 besides the subclavian veins 

 (b b) increase in size with the 

 growth of the extremities; and, 

 finally, the caliber of the two 

 jugular veins is reciprocally 

 altered, so that the rudi- 

 mentary internal jugular vein 

 becomes large (Ji}, while the 

 external jugular vein becomes 

 smaller (le) ; in many animals, 

 for example dogs and rab- 

 bits, the embryonal propor- 

 tions persist. The portion 

 of the left superior cardinal 

 vein from the point of anas- 

 tomosis down to the left duct 

 FIG. 386. -I, Rudimentary veins of the body of the embryo. II, _r p-,,: attwr^W T^~ 



Transformation of the same rudimentary into the definitive Ot |- uvier atrophies. 1 he pOS- 



veins. (Diagrammatic.) tenor cardinal veins divide 



in the pelvis into the hypo- 

 gastric (I, h) and the exter- 

 nal iliac (/ /). The inferior cava is at first small (I, V c); it divides at 

 the pelvic inlet, and passes over on each side to the point of division of 

 the cardinal veins. In addition, there exists a transverse ascending anas- 

 tomosing branch between the right and left cardinal veins. For the establish- 

 ment of the definitive condition, the inferior cava dilates (II, Ci), and with it, 

 downward, the hypogastric and external iliac on each side. The right cardinal 

 vein is replaced by the small vena azygos (A z) , and on the left side up to the trans- 

 verse anastomosing branch in an analogous manner the vena hemiazygos (Hz) . 

 Un the other hand, the upper portion above the anastomosing branch up the left 

 duct of Cuvier atrophies. The site of entrance of the vena magna cordis is the 

 remains of the left duct of Cuvier. Finally, the combined venous trunk is so 

 withdrawn into the wall of the auricle (V) that the two cavse acquire independent 

 All vertebrates possess the same rudimentary venous system in the 

 embryonal state; it persists, however, only in fishes (Fig. 74 /) 



Veins of the First and the Second Circulation, and the Development of the 

 Portal System. Originally the two omphalomesenteric veins (om, om,) empty into 

 the truncus venosus of the first pouch-shaped rudimentary heart (Fig. 387, i, #). 



