NECTURUS 75 



and the tissues; (3) the veins, which return blood to the heart; 

 and (4) the capillaries, which join the veins and the arteries. 



The Heart. The divisions of the heart have already been ob- 

 served and its ventral aspect studied (see page 67). 



The Veins. Two systems of veins are present : ( 1 ) the systemic 

 veins, which return venous blood from the various tissues and 

 organs of the body to the sinus venosus ; and ( 2 ) the pulmonary 

 veins, which return arterial blood from the lungs to the left auricle. 

 The systemic veins include (a) those which go directly to the 

 sinus venosus and their branches, and (b) the portal veins, which 

 carry blood first to the liver and kidneys, where it is distributed 

 through capillaries and afterward collected again in other veins, 

 which take it to the heart. 



We shall first study the portal veins. Two systems of these 

 veins are present: the hepatic portal system, by which blood is 

 carried from the digestive organs and the spleen to the liver ; and 

 the renal portal system, by which it is carried from the hind quar- 

 ters of the body to the kidneys. These two systems are united 

 by the abdominal vein, a prominent vessel which lies in the mid- 

 ventral line of the abdominal cavity back of the liver. 



Kill a fresh animal and skin its ventral side. Note the abdominal 

 vein, which may usually be seen through the body wall of the 

 hinder part of the abdominal cavity. Open this cavity by a longi- 

 tudinal incision to one side of the abdominal vein. 1 



Cut the mesentery which joins the liver with the ventral body 

 wall (being careful not to cut the abdominal vein, which lies in it), 

 press the sides of the body wall to the right and left, and pin 

 them there. 



The hepatic portal system is composed of the hepatic portal vein 

 and its branches and the abdominal vein, the former bringing blood 



1 Both portal systems may be injected through either the abdominal vein or 

 the mesenteric vein. If the former is used, it must be injected both forward 

 and backward; if the latter, single injection will fill both portal systems. The 

 systemic veins and the arteries should also be injected now. Turn the liver to 

 the animal's left; the very large postcaval vein will be seen coming from be- 

 tween the kidneys and entering the liver near its middle. Inject it in both direc- 

 tions. The entire arterial system may be injected through the ventricle and the 

 truncus arteriosus of the heart. 



