LATERAL DISSECTION OF THE VISCERA 107 



of the ventral aorta just caudal to the median thyreoid gland. The fourth aortic 

 arch is the largest. From the fifth arch small pulmonary arteries are developing. 

 Cranial to the first pair of aortic arches, the descending aortae are continued 

 forward into the maxillary processes as the internal carotids. Caudal to the 

 aortic arches the descending aortae converge, unite opposite the cardiac end of 

 the stomach and form the median dorsal aorta. From this vessel and from the 

 descending aortae paired dorsal intersegmental arteries arise. From the seventh 

 pair of these arteries (the first pair to arise from the median dorsal aorta) , there 

 are developed a pair of lateral branches to the upper limb buds. These vessels 

 are the subclavian arteries. From the median dorsal aorta there are also given 

 off ventro-lateral arteries to the glomeruli of the mesonephros, and median ventral 

 arteries. Of the latter the cceliac artery arises opposite the origin of the hepatic 

 diverticulum. The vitelline artery takes origin by two or three trunks caudal to 

 the dorsal pancreas. Of these trunks the posterior is the larger and persists as 

 the superior mesenteric artery. Thyng (Anat. Record, vol. 5, 1911) has figured 

 three trunks of origin in the 7.8 mm. pig. These unite and the single vitelline 

 artery branches in the wall of the yolk-sac. 



Opposite the lower limb buds the dorsal aorta is divided for a short distance. 

 From each division arises laterad three short trunks which unite to form the single 

 umbilical artery on each side. The middle trunk is the largest and apparently 

 becomes the common iliac artery. A pair of short caudal arteries, much smaller 

 in size, continue the descending aortae into the tail region. 



The Veins. — The vitelline veins, originally paired throughout, are now repre- 

 sented distally by a single vessel, which, arising in the wall of the yolk-sac, enters 

 the embryo coursing cephalad to the intestinal loop (Figs. 97, 99 and 100). Cross- 

 ing to the left side of the intestine and ventral to it, it is joined by the superior 

 mesenteric vein which has developed in the mesentery of the intestinal loop. 

 The trunk cranial to the union of these two vessels becomes the portal vein. 

 It passes along the left side of the gut in the mesentery. Opposite the origin of 

 the dorsal pancreas it gives off a small branch, a rudiment of the left vitelline 

 vein, which courses cephalad and in earlier stages connects with the sinusoids 

 of the liver. The portal vein then bends sharply to the right dorsal to the duo- 

 denum and as the right vitelline vein, passing between the dorsal and ventral 

 pancreas to the right of the duodenum, it soon enters the liver and connects with 

 the liver sinusoids. The portal trunk is thus formed by persisting portions of 

 both vitelline veins, and receives a new vessel, the superior mesenteric vein. The 

 middle portions of the vitelline veins are connected with the network of liver 



