42 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



body walls apart near the heart, so as not to injure the veins. The anterior 

 abdominal vein in the median ventral line must be preserved and should be 

 separated from the body wall before spreading the walls apart. 



i. The systemic veins (Holmes, pp. 272-74). Systemic veins are those which 

 open into the heart, returning blood from the body directly to the heart. Remove 

 the pericardial sac from the heart, and turn the heart forward so as to expose the 

 sinus venosus. Three large veins contribute to the sinus venosus: the posterior 

 vena cava, or postcaval vein, which emerges from the liver (receiving a large vein 

 on each side from the liver so that apparently three veins are present) and enters 

 the median posterior wall of the sinus; and the two anterior venae cavae, or 

 precaval veins, one opening into each lateral wall of the sinus. 



a) The precaval vein: Trace one of the precaval veins (since both have iden- 

 tical branches). It extends laterally from the heart along the border of the 

 auricle, and about i cm. away from the heart passes through the pleuro- 

 peritoneal membrane. Directly outside of this membrane it forks simulta- 

 neously into three branches. The most anterior one of these is the external 

 jugular; it passes straight anteriorly into the muscles of the floor of the mouth, 

 from which, together with the tongue, hyoid, etc., it receives venous blood 

 through numerous small branches. The middle of the three branches of the 

 precaval is the inominate; it passes laterally and then turns abruptly dorsally, 

 receiving as it turns a small subscapular vein from the muscles of the shoulder. 

 The main vein beyond the entrance of this branch is known as the internal 

 jugular; it descends straight into the hollow between the fore limb and the 

 larynx, and disappears dorsally where it collects blood from the brain and spinal 

 cord, from the roof of the mouth, and from a number of muscles. The third and 

 posterior branch of the precaval is the subclavian. It is a large vein passing 

 directly laterally to the base of the fore limb where it receives the brachial vein, 

 carrying venous blood out of the fore limb. Just beyond this point the vein, 

 now called cutaneous^ or musculo-cutaneous, turns abruptly and runs straight 

 posteriorly along the muscle of the ventro-lateral body wall (pectoral muscle). 

 It then bends sharply dorsally and passes to the skin by way of the partition 

 between the abdominal and lateral lymph sacs. It runs anteriorly under the 

 skin forward to the nares, collecting venous blood from the skin of the entire 

 dorsal side. 



b) The postcaval vein: Trace the postcaval posteriorly from the sinus venosus 

 into the liver. Where it emerges from the liver substance it receives two large 

 hepatic veins, one from the right lobe, the other from the left lobe of the liver. 

 Turn the lobes of the liver to the right and find where the postcaval enters the 

 middle lobe from behind. Trace it posteriorly. It originates between the two 

 kidneys, from which it receives a number of renal veins as well as veins from the 

 reproductive organs and fat bodies. 



