THE FROG, 183 



the arteries may usually be distinguished by their firmer 

 walls and somewhat lighter color. No visible blood vessels 

 should have been severed up to this point in the dissec- 

 tion. Begin with the venous system, and find the prin- 

 cipal avenues by which the blood is returned to the heart, 

 after being distributed throughout the tissues. Find 

 again the vein that was seen through the thin abdominal 

 wall before the body cavity was cut open. Trace this 

 vein backward far enough to see the large branches com- 

 ing up from the hind legs. Trace it also forward. Ob- 

 serve that it soon leaves the body wall, and descends 

 through the body cavity to the liver, where it branches, 

 sending one branch to each lobe of that organ. This is 

 the ventral route to the liver ; there is also a dorsal route 

 through the kidney. Turn the organs that cover the 

 kidney over to one side, and find a longitudinal vein com- 

 ing from the posterior end of the body cavity, and passing 

 along the external margin of the kidney, and dividing up 

 into numerous branches, which enter the mass of the 

 kidney. Then find a corresponding set of venous branch- 

 lets coming from the inner side of the kidney, meeting 

 branchlets from the other kidney, and uniting into another 

 large vein, which proceeds forward to the liver, receiving 

 important branches from the viscera at several points. 

 This is the portal vein. Trace it to the liver. Then 

 draw the liver backward, and turn the ventricle over for- 

 ward, and see the single large vein, the postcava, which 

 conveys the blood from the liver into the venous sinus, 

 and thence into the right auricle. Draw the heart gently 

 backward, and see the two large veins (prcecavce), which 

 bring the blood into the same chamber from the anterior 

 parts of the body. The blood flows from the auricle 

 into the ventricle, and is forced out again, by the con- 

 traction of the ventricle, through the arterial trunk. 

 Trace now its outward course. The arterial trunk 



