226 VERTEBRATES. 



already been studied, and its auricles and ventricles have 

 been located. 



I. The Venous System. Trace first the veins. The 

 right auricle receives all the venous blood from all parts 

 of the body through three principal veins, two prce- 

 cavce (right and left) and one postcava. Find these three 

 veins, and note the points at which they enter the auricle. 

 Trace each of the praecavse forward, and find each to be 

 formed by the confluence of three veins, one coming from 

 the head (the jugular), one from the wing (the brachial), 

 and one from the great pectoral muscles (the pectoral). 

 Observe that the postcava comes to the heart through the 

 right lobe of the liver, receiving small branches in that 

 organ. Posterior to the liver it is formed by the union 

 of two (iliac) veins coming from the kidneys. Three 

 other veins on each side bring the blood from the posterior 

 parts of the body to the kidneys. 



II. The Pulmonary System. The blood collected in the 

 right auricle descends through a valvular opening into the 

 right ventricle. The right ventricle then contracts, and 

 sends the blood out to the lungs through a pair of short 

 pulmonary arteries (right and left). Find these arising 

 together from the right auricle, but at once dividing, and 

 going directly one to each lung. 



From the lungs the blood, after aeration, flows back to 

 the heart through a pair of pulmonary veins (right and 

 left), which enter the left auricle separately. These will 

 probably be more easily traced from the auricle out to the 

 lungs. 



III. The Arterial System. The blood thus collected 

 into the left auricle descends through a valvular opening 

 into the left ventricle, whence it is expelled through a 

 single large artery (the aorta) to all parts of the body by 

 the contraction of the ventricle. Herein lies the explana- 

 tion of the very thick muscular walls of this ventricle. 



