. Chordate morphology. Morphology (Animals); Chordata. gradual condensation of the coeliac and superior mesenteric stems from segmental vessels can be observed. Fishes The arterial pattern of the fishes, whether dipnoan, actinopterygian, or shark does not suggest anything beyond that which is seen in the more advanced forms. In the cyclostomes there is marked contrast between the lamprey and the hagfish. In the lamprey there is a large coeliacomesenteric artery which comes off the dorsal aorta near the duct of Cuvier and, passing down to the left of the gut along with the bile duct, enters the
. Chordate morphology. Morphology (Animals); Chordata. gradual condensation of the coeliac and superior mesenteric stems from segmental vessels can be observed. Fishes The arterial pattern of the fishes, whether dipnoan, actinopterygian, or shark does not suggest anything beyond that which is seen in the more advanced forms. In the cyclostomes there is marked contrast between the lamprey and the hagfish. In the lamprey there is a large coeliacomesenteric artery which comes off the dorsal aorta near the duct of Cuvier and, passing down to the left of the gut along with the bile duct, enters the typlosole. It extends most of the length of the intestine. Far posteriorly are two or three mesenteric arteries, each enclosed by a venous chan- nel, which also serve the gut wall. In the hagfish the coeliacomesenteric passes down to the left of the gut, to the right of the bile duct, and posteriorly a short distance next to the hepatogastric mesentery. Most of the length of the intestine is served by numerous mesen- teric arteries—the primitive style as suggested in the Am- phibia. Veins Mammals In the human, blood is returned to the heart through a large postcaval vein or posterior vena cava which parallels the course of the dorsal aorta but lies be- neath that vessel. The posterior vena cava receives blood from the various hepatic vessels and, more posteriorly, a pair of renal veins and several vertebral branches; it is formed posteriorly by the union of two common iliac veins and a middle sacral vein. Each iliac has femoral and hypo- gastric branches. The pattern of the postcava and its branches is much the same as the pattern of the main arteries. The difference between these two systems lies in the fact that the veins draining the digestive tract form an hepatic portal vein. This enters the liver and branches to all parts of that structure in order to supply the small sinusoids. Blood is collected from these sinusoids by the hepatic veins and drained into the postcava.
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