342 CHORD AT A. 



lungs and the urinary bladder belong morphologically but 

 not physiologically to the alimentary system. 



The heart lies ventrally to the oesophagus, enveloped in 



its pericardium. Its structure will be con- 



lar°° Venous, sidered later. The veins lie superficially to 



the arteries and consist of an anterior and 



a posterior system. The anterior system is paired ; the 



posterior is in great part single. 



Anteriorly a small lingual vein from the tongue is seen 

 to unite with a mandibular from the lower jaw to form the 

 external jugular. This runs backwards to join with the 

 large subclavian already seen, which runs along the anterior 

 wall of the abdominal cavity. As already noticed, the sub- 

 clavian is made up of the brachial ^nd the musculo-cutaneous. 

 The area between external jugular and subclavian is drained 

 by a small but deep vein, the innominate, which is formed 

 of the internal jugular emerging from the brain and the sub- 

 scapular from the dorsal region. The innominate joins the 

 external jugular and subclavian, the three uniting to form 

 the precaval vein, which passes backwards and inwards to 

 fall into the sinus venosus dorsal to the heart. 



In the posterior system the portal vein can be seen 

 coming from the stomach, spleen, pancreas and duodenum, 

 and falling into the liver ; it constitutes the hepatic-portal 

 system. Just before it enters the liver it receives the 

 anterior abdominal vein already noted. 



If the alimentary canal be now carefully removed by 

 cutting through the rectum and through the oesophagus, 

 the kidneys are exposed and the rest of the venous system 

 is clearly distinguished (see Plate VI.). The j^;?^^ra/ veins are 

 large veins leading up from the legs. Before entering the ab- 

 dominal cavity each divides into a pelvic and a renal portal. 

 The former comes up to meet its fellow and the two form the 

 anterior abdominal to the liver. The renal portal proceeds 

 forwards, receives a sciatic from the inner side of the leg, and 

 breaks up along the outer border of the kidney ; hence the 

 frog has a well-developed renal-portal system as well as 

 a hepatic-portal. The blood from the large hind-limbs 

 must pass either through the kidney by the renal portal, or 

 through the liver by the anterior abdominal before reaching 

 the heart. Between the kidneys is a large postcaval which 



