THE CARDIAC GANGLION. 205 



polar cells, together with their dendrites, form enveloping baskets around the 

 body of the giant cells. 



c. The third kind of cells consists of small, bipolar neurones, found in the 

 first three or four segments of the heart. They are not numerous, and in some 

 cases they appear to be absent. They resemble the giant cells in shape, but are 

 smaller and are deeply stained in methylene blue. 



In the young Limuli, two or three inches long, the small cells are more clearly 

 pear-shaped and have fewer dendrites, the rounded body projecting freely from 

 the sides of the cord, to which it is attached by one or two branching processes. 



Motor Terminals. -The larger strands forming the cardiac plexus lie on 

 the outer surface of the heart; the smaller branches gradually penetrate between 

 the muscle bundles to the deeper layers, w r here one may frequently see them 

 terminate in the characteristic motor end plates. 



Sensory Terminals. Toward the anterior end of the heart, on either side 

 of the median nerve, there are peculiar, spherical masses dimly visible, imbedded 

 in the muscle layers, that probably represent free sensory terminals. Two or 

 three nerve fibers approach these bodies and form there concentric coils of fibrillae, 

 with two or three thicker vertical fibers in the center of the coils, P.c. 



It is not possible to distinguish the fibers that enter the heart from the seg- 

 mental cardiacs from those that arise in the median nerve cord. All the ganglion 

 cells increase greatly in numbers with age. 



Cardiac Ganglia in Vertebrates. The great size of the median cardiac 

 nerve of arthropods and its conspicuous origin from the overlying ectoderm led 

 me to look for a similar origin of the cardiac ganglia in vertebrates. 



In trout embryos, in frog embryos (Rana septemtrionalis) of seven millimeters, 

 and in chick embryos, from thirty to forty-eight hours, there is a thin, longitudinal 

 cord lying on the haemal surface of the heart, which resembles the median nerve 

 in the heart of Limulus, and which appears to be the anlage of the cardiac ganglion. 

 In the 30-36 hour stages of the chick, it extends along the surface of the auricles 

 and ventricles for about 150 mm., and is connected, here and there, by swollen 

 strands, with the overlying ectoderm. 



I was not able to follow in a satisfactory manner the history of this structure, 

 but it seems to me probable, from its general appearance and location, that under 

 suitable conditions it will be possible to trace its development into the cardiac 

 ganglia of the adult. 



VI. EXPERIMENTS ON THE HEART. 



In my notes from the summer of 1897, I find records of observations to the 

 effect that the isolated heart continued to beat for many hours, either in salt solu- 

 tion, or in a moist chamber; that small segments of the heart would continue to 

 beat, provided a piece from the posterior part of the median ganglion was at- 

 tached; and that separate pieces that did not contain this part of the median nerve, 



