THE FROG HEART. 129 



surface, which indicates the moment of excitation. Take three or four similar 

 curves, depressing the table an equal distance after each observation ( or 

 turn) by the handle. Remove the muscle lever and take a tracing with a tuning- 

 fork, vibrating 100 times a second, carefully arranging the style of the fork in 

 the position previously occupied by the writing end of the muscle lever. 

 Remove the paper, varnish and measure the tracings. From the mean result 

 of the measurements, the latent stimulation may be computed. 



8. Rate Of Propagation- I. Prepare the muscle as in the last exercise. 

 Expose the sciatic nerve throughout its length. Place one pair of electrodes 

 under the nerve close to the muscle, and a second pair under the nerve near its 

 origin. Connect these two pairs of electrodes with a switch. To the middle 

 screws of the switch attach the wires from the secondary coil, so that by turn- 

 ing over the bridge of the switch, the near and the distant portion of the 

 nerve can be excited alternately without loss of time. The nerve should be 

 prepared with great care, and each exposed part should be protected by a flap 

 of muscle, except at the moment that it is being excited. Take tracings of 

 muscle curves in pairs, alternately exciting the near and distant portions of the 

 nerve. Take a tuning-fork tracing, varnish, measure the length of nerve 

 from one pair of electrodes to the other, and therefrom determine the rate of 

 propagation in the nerve (see p. 77). 



III. The Frog Heart. 



1. Rhythmical Motions- In a curarized preparation of which the 

 hemispheres have been destroyed, expose the sternum and cut across the 

 episternal cartilage. Then sever the sternum from its connections by a cut on 

 either side, and turn it down over the belly. The heart is seen still covered 

 by the pericardium. Expose the heart by carefully dividing the peri- 

 cardium. Note the condition of each of its cavities and the mode of its 

 rhythmical action. 



2. The Inhibitory Centre. For the purpose of observing the effect 

 of passing series of induction shocks through the inhibitory centre of the 

 heart, a fine ligature is attached to the frsenum (the thread-like ligament which 

 stretches from the dorsal aspect of the ventricle towards the lower part of the 

 pericardium). By means of the ligature the heart is raised out of its place 

 and turned upwards. The inhibitory centre is recognized by the whitish 

 crescent -shaped line which marks the junction of the wall of the sinus with 

 that of the right auricle. Faradize this spot for a second or less, placing the 

 points of the electrodes on the line, a couple of millims. distant from each 

 other. Observe the mode and order in which the cavities of the heart resume 

 their rhythmical action. 



3. Destroy the spinal cord by pithing, and observe the changes thereby 

 produced in the state of the circulation, and particularly in the mode of action 

 of the heart. 



4- The Cardiac VagUS Of the Prog. a. Preliminary Dissection. 

 Expose the trunk of the vagus nerve as it escapes from the cranium as follows : 

 Remove the integument so as to bring into view the muscles of the back of 



K 



