THE HEART 87 



Tighten the ligatures upon both vagi nerves. Apply a second liga- 

 ture to each at a distance of 2 mm. from the first. Cut between them. 

 Stimulate both central ends successively. Do you observe a change in 

 the character of the record now made? Stimulate both distal ends 

 successively. Do you observe a difference between this record and 

 that obtained previously with the intact nerve? 



Apply the electrodes transversely to the sino-auricular region of 

 the heart. Stimulate after you have recorded a limited number of 

 normal beats. Does the inhibition produced in this way differ from that 

 previously obtained by stimulating the vagi nerves? 



Annotation. A very convenient way is to trephine the ventral shield of the 

 turtle in the region of the heart. This saves much labor and prevents loss of blood 

 and drying of the tissues. Moreover, if small pulley-wheels are at the disposal of 

 the students, the heart should be allowed to act in its normal horizontal position, 

 while the string is made to move across the pulley, placed obliquely below the 

 writing lever. Do not allow the heart to act under too great a tension and allow 

 it to rest from time to time by disconnecting the string. In case it should cease 

 contracting properly, apply a few drops of warmed saline solution to its surface. 

 Arhythmias are not uncommon and may be remedied in just this w r ay. 



2. Action of Nicotin. By means of a dropper apply a small quantity 

 of a 0.2 per cent, solution of nicotin to the heart. After five minutes 

 connect its apex with the writing lever and stimulate the vagus nerve. 

 The heart is not inhibited. Stimulate the sino-auricular region directly. 

 The heart is inhibited. What is your conclusion regarding the action of 

 nicotin? 



Annotation. Nicotin is a nerve-cell poison affecting the neuron at the synapse. 

 In this case it causes a break between the vagal terminals and the recipient cells 

 of the inhibitor plexus (Remack's), situated in the region of the sino-auricular groove. 

 Consequently, the inhibitor impulses set up by stimulating the vagus can never reach 

 the postganglionic path and effector. The stimulation of the plexus itself remains 

 effective, because the postganglionic elements are thereby excited directly. 



3. Action of Atropin. Apply to the heart of the same turtle a few 

 drops of a 0.5 per cent, solution of atropin sulphate. After a few minutes 

 stimulate the plexus situated at the sino-auricular junction. Observe 

 that this stimulation now fails to inhibit the heart. Explain. 



Annotation. This experiment should, of course, be performed upon a fresh 

 turtle. It will then be found that the atropin destroys the inhibitor power of the 

 vagus as well as that of the plexus at the sino-auricular groove. This result is due to 

 its paralytic effect upon the distal terminals of the postganglionic fibers. Atropin is 

 primarily a nerve-fiber poison. Since in the above experiment the stimulation of 

 the vagus has already been rendered ineffective by the nicotin, this effect cannot be 

 noted. It is for this reason that a fresh turtle should be used for the atropin. With 

 a class of, say, 100 students this would entail a considerable expense which may be 

 avoided by supplying this information or by permitting one group of students to per- 

 form the experiment with nicotin, and another the experiment with atropin. 



4. Action of Muscarin. Apply to the ventricle of the same turtle 

 a few drops of normal saline to which a little muscarin has been added. 



