COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 225 



may pass into the abdominal vein and so into the hepatic portal vein, or may 

 pass into the renal portal vein. This arrangement appears to be a device to 

 assist the return of the blood from the posterior regions of the body. In the 

 elasmobranchs there is no such connection between the two portal systems 

 (although connections have been reported in some specimens as individual 

 variations). Further, in elasmobranchs the lateral abdominal veins enter the 

 cardinal system of veins, while in Necturus their homologue, the ventral abdomi- 

 nal vein, empties into the hepatic portal system (Fig. 57). 

 4. The systemic veins. — 



a) The anterior systemic veins: It has already been noted that the common 

 cardinal vein joins the hepatic sinus on each side in the pericardial cavity. 

 Turn to the pericardial cavity and locate the common cardinal veins. Trace 

 one of them laterally, removing the muscles between the pericardial cavity and 

 the base of the fore limb. Just outside of the pericardial cavity the common 

 cardinal receives the jugular and subclavian veins. The latter lies ventral and 

 anterior to the former. Trace the subclavian into the fore limb by removing 

 the skin from the outer surface of the limb. The subclavian is seen to be formed 

 at the shoulder by the union of the cutaneous vein from the skin and the brachial 

 vein which runs along the surface of the limb muscles. The jugular vein is 

 homologous with the anterior cardinal vein of the elasmobranchs. Follow it 

 forward. It is formed by the union of the external and internal jugular veins. 

 The external jugular vein first receives branches from the floor of the mouth. 

 The main vein then passes dorsally immediately behind the gills. It may be 

 picked up here by removing the skin behind the last gill. The external jugular 

 may then be traced forward above the gills, where it enlarges, forming the 

 jugular sinus. This sinus receives tributaries from the head and jaws. The 

 internal jugular vein is a small vein which joins the external jugular posterior 

 to the jugular sinus. It is difficult to find, and part of it will be seen later in 

 the roof of the mouth. 



The common cardinal vein also receives a lateral vein from the body wall. 

 Remove the skin from the lateral line shortly posterior to the fore limb. Cut 

 through the shoulder muscles so as to reveal the partition (horizontal skeletoge- 

 nous septum) between the epaxial and hypaxial muscles. The lateral vein will 

 be found situated along this partition and can be followed forward into the 

 common cardinal vein. 



b) The postcaval vein: Turn to the pleuroperitoneal cavity. Examine the 

 dorsal mesentery of the small intestine at its junction with the dorsal body wall 

 (in female specimens spread the ovaries apart, laying one to each side). In the 

 mesentery runs a large vein, the postcaval vein. It passes forward, receiving 

 numerous genital veins from the adjacent gonads and renal veins from the kidneys. 

 Trace it forward. At about the level of the spleen it turns ventrally and enters 

 the dorsal surface of the right side of the liver. It is best seen by laying the 



