CIR CULA TION. 1 49 



endocardium. Branches from these plexuses form :i third plexus in the myo- 

 cardium or heart muscle, from which arise a vast number of non-medullated 

 terminal nerves, enveloping the muscle-fibres and ending in small enlargements 

 or nodosities of various forms. Similar ,% varicose " enlargements are observed 

 along the course of the nerves. The nerve-endings are in contact with the 

 naked muscle-substance, the mode of termination resembling in general that 

 observed in non-striated muscle. Ganglion-cells are found chiefly in the 

 auricular septum and the auriculo-ventricular furrow, but arc present also 

 beneath the pericardium of the upper half of the ventricle. Xo ganglia have 

 as yet been satisfactorily demonstrated within the apical half of the ventricle, 1 

 and most observers do not admit their presence within the ventricular muscle 

 itself. The nerve-cells are unipolar, bipolar, or multipolar. 



Certain unipolar cells in the frog are distinguished by a spherical form, a 

 pericellular network, and two processes — namely, the axis-cylinder or straight 

 process, and the spiral process. The latter is wound in spiral fashion about 

 the axis-cylinder, ending in the pericellular net. According to Retzius and 

 others, the spiral is not really a process of the cell, but arises in a distant extra- 

 cardiac cell and carries to the heart-cell a nervous impulse which is transmitted 

 from the spiral process to the cell by means of the contact between the peri- 

 cellular net and the cell-body. Section of the cardiac fibres of the vagus 

 causes the spiral " process " and pericellular net to degenerate, the cell-body 

 and axis-cylinder process remaining untouched, showing that the spiral process 

 is the terminal of a nerve-fibre running in the vagus trunk. 2 



Nerve-theory of Heart-beat. — The theory of the nervous origin of the 

 heart-beat rests in part on the correspondence between the degree of contrac- 

 tility of the various parts of the heart and the number of nerve-cells present 

 in them. Thus the power of rhythmical contraction is greater in the auricle, 

 in which there are many cells, than in the ventricle, in which there are fewer. 

 The properties of the apical half, or "apex," of the ventricle are considered 

 to be of especial importance in the study of this problem, because the apex, as 

 has been said, is believed to contain no ganglion-cells. This part of the ven- 

 tricle stops beating when separated from the heart, while the auricles and the 

 ventricular stump continue to beat. The apex need not be cut away in order 

 to isolate it. By ligating or squeezing the frog's ventricle across the middle 

 with a pair of forceps the tissues at the junction of the upper and the lower 

 half of the ventricle can be crushed to the point at which physiological con- 

 nection is destroyed but physical continuity still preserved. Such frogs have 

 been kept alive as long as six weeks. The apex does not as a rule beat again. 

 The exceptions can be explained as the consequence of accidental stimulation. 

 The conclusion drawn Is that the apex, in which ganglion-cells have not been 

 satisfactorily demonstrated, has not the power of spontaneous pulsation which 



'Schwartz: Archiv fur mikroskopiache Anatomie, 1899, liii. S. 63. Compare Dogiel : Ibid., 

 S. 237. 



2 Nikolajew: Archiv fur Physiologie, 1893, Suppl. Bd., S. 7.".. 



