116 PRACTICAL PHYSIOLOGY 



in the root of the aorta, and run in the auriculo-ventricular groove. The 

 left is the more important, and divides into the ramus circumplexus 

 and the ramus descendens. Ligation of the left coronary artery causes 

 fibrillar contraction, ending in arrest of the heart beat. Ligation of 

 either of its chief branches may cause fibrillar contraction. Trace the 

 coronary vein in the auriculo-ventricular groove. In the posterior 

 wall of the right auricle find the opening of the vena cava inferior 

 below, and that of the superior vena cava above. 



In the posterior wall of the left auricle note the openings of the two 

 pulmonary veins in man there are four. The pulmonary artery arises 

 in front of the base of the right ventricle, close to the anterior intra- 

 ventricular groove. The aorta arises from the base of the left ventricle 

 behind and a little to the right of the pulmonary artery. Tie a glass 

 tube V.C. into the vena cava superior, and ligature the vena cava 

 inferior and the left vena azygos. Tie a second glass tube P.A. into 

 the pulmonary artery. The arterial tube should be two feet, and the 

 vena cava tube one foot long. Fill the V.C. tube with water, the water 

 runs through the right ventricle into P.A. Ehythmically compress the 

 ventricles with your hand. The water sinks in V.C., and rises in P.A. 

 Stop the compression. If the pulmonary valve is competent, the fluid 

 in P.A. will not sink. Note that the root of the pulmonary artery is 

 distended by the pressure of the column of fluid. The same experiment 

 can be repeated, using one pulmonary vein and the aorta. Remove the 

 tubes, and measure the diameter and compare the sectional areas of the 

 two venae cavae, the pulmonary artery and the aorta. Extend the 

 aorta and pulmonary artery. Both have extensile and elastic walls, 

 and although empty, do not collapse. The pulmonary artery is very 

 extensile. The venae cavae and pulmonary veins have thin inextensile 

 walls which fall together. 



Pass your finger down the superior vena cava through the right auricle 

 into the right ventricle. Feel the size of the auriculo-ventricular orifice. 

 Now cut through the pulmonary artery just above its origin, and look 

 within. Note the three pulmonary semi-lunar valves. Put the pulmonary 

 orifice under the tap. The pocket-shaped valves close and prevent the 

 water entering the ventricle. Pass a finger through the pulmonary 

 orifice, and another through the superior cava. The two fingers meet 

 in the right ventricle. 



Next cut open the right auricle, and observe that it surmounts the 

 right auriculo-ventricular orifice like an inverted pocket. 



Note the appendix with its fretwork of muscle the inter-auricular 

 septum with the fossa ovalis ; the Eustachian valve, a membraneous 

 fold in low relief, which lies immediately beneath the entrance of 



