26 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIC. 



terior cardinal vein) is in reality a part of the prevertebral plexus and is more medial in 

 position than the posterior cardinal vein ever is. 



The diagram of figure 14 is from an embryo 24 mm. long and shows that the lower 

 segment of the inferior vena cava is symmetrical up to this stage. By the time the embryo 

 is 28 mm. long a very extensive anastomosis across the mid-line dorsal to the bifurcation 

 of the aorta allows a shifting of the blood of the left side to an enlarged trunk on the right 

 side and thus the asymmetrical right trunk of the postrenal inferior vena cava and the 

 ascending lumbar veins are formed. 



The significance of these changes associated with the prevertebral plexus lies in the 

 decreasing circulation of blood through the Wolffian bodies all along the line. In the 

 thoracic region the development of the azygos system in the prevertebral plexus has resulted 

 in a shunting of the blood of the spinal and intercostal veins away from the cardinal system 

 and in the abdominal region a much more extensive prevertebral vascular plexus accom- 

 plishes the same purpose. In the abdominal region this prevertebral plexus is certainly 

 of great physiological significance. In the first place, it drains practically entirely into the 

 part of the Wolffian body which belongs to the vena cava, and hence it might be called a 

 prevertebral caval plexus. Its beginning is shown in a sheet of vessels in figure 7, from an 

 embryo 14 mm. long, and still better in figure 13, from an embryo 17 mm. long, in which 

 it can be seen that the plexus of vessels coming from the body-wall is very much more 

 extensive than the spinal or segmental veins. 



Figure 12 may be taken as a typical section of the prevertebral plexus of the abdominal 

 region. By comparison with figure 10, which gives the prevertebral plexus of the thoracic 

 region, it becomes clear that the abdominal plexus is an extraordinarily complex one. It 

 fills in the entire region between the aorta and the vertebrae on the one hand, and between 

 the aorta and the Wolffian bodies on the other, so that it is not only dorsal to the aorta but 

 lateral as well. In the thoracic region it is the more medial part of the plexus which is 

 best developed (fig. 10), while in the abdominal region it is the more lateral part (fig. 12). 

 That the lateral part of the plexus is more important throughout the abdominal region 

 is clearly indicated in the beginning of its development (as seen in figure 13), in which 

 the two sheets of veins distinctly lateral to the azygos system have already been described. 

 In this plexus the azygos and hemiazygos veins have a characteristic position, directly 

 dorso-lateral to the aorta (fig. 10), while the developing vena cava is farther lateral. 

 From the whole history of the azygos system it is clear that there is a prevertebral plexus 

 throughout the embryo. In the abdominal region this prevertebral plexus becomes 

 excessively complex. It may well be divided into three longitudinal zones a zone cephalic 

 to the median mesonephritic vein, a zone including the mesonephritic vein (fig. 12), and 

 a zone caudal to the same vein. In the first zone, just cephalic to the median vein, the 

 adrenal bodies develop in the area just lateral to the aorta. They, therefore, develop in 

 an excessively vascular area. In the third abdominal zone, caudal to the median vein, the 

 lower segment of the inferior vena cava develops also lateral to the aorta. 



This history, then, enables one to complete the analysis of the inferior vena cava from 

 the heart downward. The segment above the liver comes from the omphalo-mesenteric 

 veins; within the liver from the omphalo-mesenteric veins and certain hepatic sinusoids; 

 below the liver from the right mesial cardinal vein of the Wolffian body with its great 

 median anastomosis; and below the kidneys from the prevertebral plexus. It thus becomes 

 clear that the posterior cardinal vein, which early became incorporated in the Wolffian 

 bodies, does not persist as a part of the vena cava. In fact, of the cardinal system below 

 the duct of Cuvier, the only part that remains is the median anastomosis of the mesial 



