224 VASCULAR SYSTEM 



vessels the arteries to the posterior extremities and the vessels to the pelvic viscera 

 arise. 



Arteries of the limbs. The limb -arteries are first laid down in the limb-bud 

 in the form of a plexus of capillary loops (see fig. 223, p. 178). This plexus arises 

 probably not from a single vessel, but from several representing a segmental 

 series. E. Miiller has described a plexus in the developing upper limb related to 

 the nerves of the brachial plexus which he regards as being suggestive of a primitive 

 segmental arrangement, but he did not observe a stage in which there was more 

 than one subclavian artery. ] As the nerves extend into the growing limb-buds, the 

 capillary loops follow mainly the course of the nerves (De Vriese, E. Miiller 2 ), 

 and the definitive arteries are formed by the enlargement of certain of these loops. 

 The main arterial stems are at first central, but later the lateral branches assume 

 larger proportions, and the primitive arrangement is lost. In the arm the primary 

 brachial stem is continued to the forearm and hand, as a mesial vessel (in part 

 the future anterior interosseous) which pierces the carpus and extends to the 

 dorsal aspect of the hand. This artery soon recedes in importance, and the loop 

 accompanying the median nerve becomes the main artery of the forearm. This 

 in turn becomes a secondary stem owing to the enlargement of lateral loops which 

 become the ulnar and radial arteries. The original artery of the lower limb 

 accompanies the sciatic nerve, but a new vessel represented by the external iliac 

 and the femoral appears later. This anastomoses above the knee with the sciatic 

 artery, and then becomes the main artery of the limb, the original vessel becoming 

 reduced to the arteria comes nervi ischiatici. The primary stem is continued 

 to the leg and foot as an interosseous artery, which, like that of the arm, pierces 

 the tarsus and reaches the dorsum of the foot. It is represented in part by the 

 peroneal artery. The anterior and posterior tibial arteries are secondary loops enlarged 

 to form the main arteries. 



DEVELOPMENT OF THE PRINCIPAL VEINS. 



At the fifteenth day, as we have already seen (p. 63), the ductus venosus receives 

 three veins on each side, the vitelline from the yolk-sac, the umbilical (allantoic) 

 from the body-stalk, and the ducts of Cuvier, formed by the union of the anterior 

 and posterior cardinal veins . 



Vitelline veins. The vitelline or omphalo-mesenteric veins enter the abdo- 

 men along the vitelline duct and ascend at first along the front of the alimentary 

 canal, but higher up they come to lie on either side of that tube (duodenum). 

 Here transverse communications form between the two veins, two in front of, and 

 one behind the duodenum, so that the gut is encircled by two vascular rings 

 (figs. 279, 280). Above these venous circles the direct communication with the sinus 

 becomes lost, the intermediate venous vessel on either side becoming broken up 

 within the substance of the liver (which has by this time developed around them) 

 into a vascular network. The vascular network is produced by the breaking up 

 of the large vessels by the hepatic epithelial cylinders which grow into and cut 

 up the lumen into a network of capillary-like spaces (sinusoids, Minot). 



The vessels which pass from the upper venous ring to the liver sinusoids are 

 known as vence advehentes : they become the branches of the portal vein ; those 

 which pass from it into the sinus are the vence revehentes : they become the hepatic 

 veins. 



1 That the limb- arteries are in the first instance segmentally arranged was on theoretical grounds 

 suggested many years ago by Macalister (Journ. of Anat. and Phys. xx. 1886) and by Mackay 

 (Memoirs and Memoranda in Anatomy, Cleland, i. 1889). Hans Rabl has recently brought forward 

 some objective evidence in favour of this hypothesis (Arch. f. mikr. Anat. Ixix. 1907). 



a De Vriese, Bertha, Arch, de Biologic, xviii. xxi. ; E. Miiller, Anat. Hefte, xxii. xxvii. ; see also 

 Gb'ppert, Ergebnisse der Anat. und Entwickelungsgeschichte, xiv. 1905. 



