|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
|arterial stem for the upper arm, the direct continuation of this stem in the forearm is the volar interosseous artery. A branch which accompanies the median nerve soon increases in size and forms the main vessel (median artery) of the forearm, while the volar interosseous diminishes. Later the radial and ulnar arteries are developed as branches of the brachial part of the stem and coincidently with their enlargement the median artery recedes; occasionally it persists as a vessel of some considerable size and then accompanies the median nerve into the palm of the hand.|
| The primary arterial stem for the lower limb is formed by the inferior gluteal (sciatic) artery, which accompanies the sciatic nerve along the posterior aspect of the thigh to the back of the knee, whence it is continued as the peroneal artery. This arrangement exists in reptiles and amphibians. The femoral artery arises later as a branch of the common iliac, and, passing down the front and medial side of the thigh to the bend of the knee, joins the inferior gluteal artery. The femoral quickly enlarges, and, coincidently with this, the part of the inferior gluteal immediately above the knee undergoes atrophy. The anterior and posterior tibial arteries are branches of the main arterial stem.|
FIG. 475 The liver and the veins in connection with it, of a human embryo, twenty-four or twenty-five days old, as seen from the ventral surface. (After His.) (See enlarged image)
Further Development of the Veins.The formation of the great veins of the embryo may be best considered by dividing them into two groups, visceral and parietal.
The Visceral Veins.The visceral veins are the two vitelline or omphalomesenteric veins bringing the blood from the yolk-sac, and the two umbilical veins returning the blood from the placenta; these four veins open close together into the sinus venosus.
| The Vitelline Veins run upward at first in front, and subsequently on either side of the intestinal canal. They unite on the ventral aspect of the canal, and beyond this are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of the intestine, which is thus encircled by two venous rings (Fig. 475); into the middle or dorsal anastomosis the superior mesenteric vein opens. The portions of the veins above the upper ring become interrupted by the developing liver and broken up by it into