EXPERIMENTS. 207 



segmental and the pericardial nerves gave no results, d. The median cardiac 

 nerve was now cut (transected) about one-third the distance from the anterior 

 end of the heart about in the middle of the third cardiomere. Rhythmic pulsation 

 of that part of the heart in front of the cut stopped for half an hour. At the end 

 of this time the anterior part began to beat again, e. Dissected out, and removed, 

 a piece of the median cardiac nerve, about three-fourths of an inch long, from the 

 anterior end of the posterior part left after the first operation. All pulsations of 

 the section of the heart without any median -nerve (i.e. y the fourth cardiomere, 

 approximately) immediately ceased, while the parts in front and behind con- 

 tinued to beat rhythmically. After an hour, the fourth cardiomere recovered 

 and began beating with a rhythm of its own, distinct from that of the posterior 

 part of the heart. /. Stimulation of the lateral nerve of the fourth cardiomere, 

 about one hour after operation e, caused one strong contraction of that part of the 

 heart instead of the increased beat, as before. Stimulation of the lateral nerve 

 of the posterior cardiomeres gave the same results as before, i.e., acceleration and 

 augmentation of the beat. g. Stimulation of the haemal nerves of the branchial 

 neuromeres, or of the branchial nerves, or of the ventral cord itself, produced no 

 effect on the heart beat. Stimulation of a lateral cardiac nerve, after removal of 

 the median nerve, caused a contraction on the stimulated side only. 



B. Rhythm of Heart Beat after Stimulation. Counts were made of the 

 number of beats per minute of the normal unstimulated heart, and also during 

 stimulation of the various nerves. The results will be presented in tabular form. 



a. Unstimulated, 32 beats per minute. 

 Electrodes on the median nerve, 24 beats per minute. 



Electrodes on a lateral branch of the median nerve, 12 beats per minute. 

 Electrodes on a lateral branch of the median nerve, 14 beats per minute. 

 Stimulation of the lateral cardiac nerve caused a contraction of the side of the 

 heart stimulated, but did not affect the rhythm of the opposite side. 



b. Unstimulated, 24 beats per minute. 



Electrodes on median nerve (near middle of heart) 8 beats per minute. 

 Electrodes on median nerve (posterior end) , 20 beats per minute. 

 Electrodes on lateral branch of median nerve, 6 beats per minute. 



c. Placing the electrodes underneath the median nerve near the middle of 

 the heart, inhibits the beat of the whole heart. 



d. Placing the electrodes so far as possible on the muscles only of the heart, 

 does not perceptibly affect the beat. Stimulation of the anterior abdominal 

 nerves, or the abdominal neuromeres does not cause any perceptible change in the 

 rhythm. 



Carlson (Am. Journ. Physiol., 1894 and 1905) has made more elaborate ex- 

 periments on the heart of Limulus than we have, but so far as our experiments and 

 his overlap, they are in agreement on the most important points. Carlson, how- 

 ever, was not aware of certain details in the histological structure of the median 

 nerve, as indeed we were not at that time, which are essential to the interpretation 



