XIII.] STRUCTURE AND ACTION OF THE HEART. 137 



'2. Tie a short glass tube into the superior vena 

 cava and connect with it a piece of india-rubber 

 tubing. Ligature the inferior vena cava and the 

 left vena azygos which opens close beside it. 

 Tie a glass tube about two feet in length into 

 the pulmonary artery. Fill the india-rubber 

 tubing with water, and squeezing it press the 

 water onwards. The water will amount in the 

 tube connected with the artery, and will only 

 descend a little way on unclasping the india- 

 rubber tubing. Pour water into the long glass 

 tube by means of a funnel, and observe the 

 column of water which the semilunar valves will 

 sustain. Note the distension of the arterial 

 walls and the bulging at the attachment of the 

 valves. When the pressure of the column of 

 fluid is removed the artery by its elasticity 

 returns to its previous dimensions. 



3. Repeat the above observation with the pul- 

 monary veins and aorta. 



4. Compare the united sectional areas of the 

 superior and inferior venae cavae when distended, 

 with the area of the aorta below the origin of 

 the innominate artery. 



5. Having removed the tubes, lay open the superior 

 and inferior venae cavae, and bring the incisions 

 to meet in the front of the auricle. Note 



a. The size and form of the auricular cavity. 



b. The auricular appendage with its muscular 

 fretwork. 



