LESSON XIX. THE DIGESTIVE AND EXCRETORY 



SYSTEMS. 



1. Cessation of the circulation leads to a rapid de- 

 crease in the effect of most autonomie nerves. In con- 

 sequence when the central nervous system has been 

 destroyed in a frog, it is necessary to dissect the nerves 

 quickly. The first stimuli should be for 1-2 sees, only, 

 and the stimulus should cease immediately the contrac- 

 tion begins. ^ 



The circulation will continue if the fore- brain only is destroyed. 

 In this case it is best to inject into the dorsal sac the minimal amount 

 of curari required to paralyse the motor nerves (about J c.c. of -02 p.c. 

 solution). The blood vessels which it is necessary to cut in the dis- 

 section should first be tied. 



Arrange the inductorium for tetanising currents of a 

 strength just sufficient to be felt on the tongue. Thread 

 a small ligature needle (instead of a ligature needle two 

 fine pointed forceps may be used) for passing the thread 

 under the nerves. Note the position of the bladder in 

 the frog dissected for you. 



In a just pithed frog cut through the abdominal wall 

 in the mid line, and carry the cut up through the ster- 

 num. Make two transverse cuts in the abdominal wall 

 midway, and pin back the lower flaps. Pin the legs 

 firmly. Turn the viscera to one side and pin them back. 

 The nerves of the lumbo-sacral plexus will be seen 

 through a thin membrane, cut this through to expose 

 the nerves. Pass a thread under the 9th nerve (the 

 lowest large one). Cut through the ramus communicans 

 of the 8th nerve (cp. Fig. 28), tie the 9th nerve near its 

 L. 7 



