LATERAL DISSECTION OF THE VISCERA 



109 



The anterior cardinal veins are formed by the plexus of veins on each side 

 of the head which are drained by two trunks (Figs. 98 and 99). These extend 

 caudad and lie lateral to the ventral portion of the myelencephalon. Each an- 

 terior cardinal vein receives branches from the sides of the myelencephalon, then 

 curves ventrad, is joined by the linguo-facial vein from the branchial arches and 

 at once unites with the posterior cardinal of the same side to form the common 

 cardinal vein. This, as we have seen, opens into the sinus venosus. 



Spinal cord 

 Anterior cardinal vein 

 Cervical sinus 



Pericardia! cavity 



R. common cardinal vein 



Post, cardinal vein 

 Esophagus 



Large Tenons sinusoid Liver 



Anterior limb bud 



Inf. vena, cava 



Post. Post, cardinal vein 



Mesonephros (cut surface) 



R. subcardinal vein 



Venous sinusoid on 

 dorsum of mesonephros 



Dorsal aorta, 

 Nolochord 



Notochord 



Trachea 



L. common cardinal vein 

 Lung 



Liver 



Stomach (cut edge) 



Omental bursa 



Mesogastrium 



Mesonephros (cut surface) 



Capillary anastomosis between 



subcardinal veins 



Vitelline artery in dorsal 



mesentery 



Capillary anastomosis between 



subcardinal veins 



Venous sinusoid on dorsum 

 of mesonephros 



Spinal cord 



FIG. 98. Reconstruction of the cardinal and subcardinal veins of a 6 mm. pig embryo showing the early 

 development of the inferior vena cava (K. L. Vehe). 



The posterior cardinal veins develop on each side in the mesonephric ridge, 

 dorso-lateral to the mesonephros (Figs. 98 and 99). Running cephalad, they 

 join the anterior cardinal veins. When the mesonephroi become prominent, as 

 at this stage, the middle third of each posterior cardinal is broken up into sinusoids 

 (Minot). Sinusoids extend from the posterior cardinal vein ventrally around 

 both the lateral and medial surfaces of the mesonephros. The median sinusoids 

 anastomose longitudinally and form the subcardinal veins, right and left. The 



