246 THE PIGEON 



Split open the gizzard and notice : 



(a) The thickness of its muscular walls. 



(b) The tough cuticular lining of the cavity and the small stones within it. 



The gizzard is therefore a mill for grinding the contained food. 



(c) The openings of the proventriculus and duodenum which are situated 



dorsally and near together. 



The Vascular System. 



Examine the large conical four-chambered heart in the pericardium. Note 

 the right and the left auricles which form the thin- walled dark-coloured anterior 

 portion, and the right and left ventricles forming the posterior thick-walled paler 

 portion. A line of fat on the outside indicates the division between the two 

 auricles. The left ventricle is hidden from view, being partly encircled by the 

 right ventricle ; the former has a circular cavity and the latter a crescentic 

 cavity in transverse section. There is no sinus venosus. 



Make a section across the posterior portion of the heart so as to show the 

 above arrangement of the ventricles. 



A. The Venous System. 



Notice that the right and the left anterioj.' vena cava (precaval veins) and 

 the single posterior vena cava (postcaval vein) open into the right auricle. 



Each anterior vena cava or precaval vein is formed by the union of the 

 following veins : — 



(a) The external jugular vein running along the side of the neck close to the 



carotid artery and alongside the vertebral column. Anteriorly the 

 jugulars meet in the region of the gullet. 



(b) The brachial vein from the wing, underlying the brachial artery. 



(c) The pectoral vein from the pectoral or breast muscles. 



The posterior vena cava or postcaval vein is formed by the union of the two 

 common Uiac veins. It passes through the tissues of the liver forwards to the 

 right auricle of the heart ; and receives the hepatic veins as it issues from the 

 hver. The caudal vein from the tail branches into two " renal portal " or 

 hypogastric veins. Each receives a small internal iliac vein and passes on 

 through the substance of the kidney, there receiving a sciatic and a femoral vein 

 from the leg and then issuing frorn the kidney as the conunon iliac vein. The 

 common iUacs receive small renal veins from the kidneys. At the origin of the 

 " renal portal " veins there arises the coccygeo-mesenteric vein ; it collects 

 blood from the cloacal region and the large intestine and runs forward to join 

 thfe hepatic portal vein, which is formed by the union of gastro-duodenal, 

 anterior and posterior mesenteric veins and which enters the liver. The 

 epigastric vein (from the omentum) joins the left hepatic vein. (See Fig. 86, 

 p. 244.) 



Note that the impure blood passes from the right auricle into the right 

 ventricle. The pulmonary artery, which arises from the left side of the right 

 ventricle anteriorly, conveys the blood to the lungs. The purified blood 

 returns by the pulmonary veins, very short vessels which enter the left auricle 

 dorsally. Thence the blood passes into the left ventricle, and is pumped to the 

 body through the single aortic arch. 



