PRACTICE DISSECTIONS 143 



num open endwise, and make the following observations : 

 Isolate and trace out both phrenic nerves. Trace the 

 vago-sympathetic trunks from the neck region down into 

 the chest. Follow the right nerve closely in the tissues at 

 the apex of the chest. At this point isolate the subclavian 

 vein and tie two ligatures tiglitly around the vessel. Be 

 sure tlie ligatures are tied iigl/itly enough not to slip off. 

 Cut the vein between the ligatures and follow the nerve 

 down behind the vein to the subclavian artery. (See Figs. 

 133 and 184.) Isolate the annulus Vieussenii (ansa sub- 

 clavia) and pick up the fibers that pass off toward the 

 heart. Part of these fibers are from the vagus proper and 

 if stimulated in the living animal will S!OW T or weaken (or 

 stop) the beating of the heart. A few of the remaining 

 fibers are from the sympathetic system (Figs. 148 and 318) 

 and when stimulated in the living animal will cause the 

 heart to beat faster (accelerators) or stronger (augmen- 

 tors). Could you perform a dissection like this in a living 

 animal? What would be the most likely causes of failure? 

 How could you avoid these? Open the pericardium, ex- 

 amine the auricles and ventricles. Isolate the left pul- 

 monary artery and pass a ligature beneath it. Hunt for 

 the thoracic duct. Where is it located in the chest? What 

 does it resemble? With what might you confuse it? The 

 writer has seen students look in the wrong side of the 

 chest for the duct. What criticism would you offer in 

 such a case? 



EXPERIMENT XIII. 



Whiskey or Brandy. (Reaction Time.) 



1. Observe carefully the arrangement of the apparatus 

 shown in Fig. 134. The subject of the experiment holds 

 down (closed) the handle of key A; key B remains open. 

 The writing point of the signal magnet does not move. 



