NERVES OF THE HEART. CARDIAC MUSCLE. 257 



A single nerve, N, goes to the heart from each side (only 

 that of the right side is represented in the diagram). This 

 nerve is usually spoken of as the cardiac branch of the vagus 

 or pneumogastric, P, but it is partly made up of fibres from 

 the sympathetic nerve, S, which join the pneumogastric close 

 to the skull and run on with its cardiac branch, the two form- 

 ing the apparently single nerve-trunk, N, which runs to the 

 venous sinus, breaking up near it into several twigs. On these 

 twigs and in the plexus which they form in the wall of the sinus 

 are numerous nerve-cells, forming the sinus ganglion or 

 ganglion of Remak. From the sinus nerves run down the 

 walls of the auricles to the auriculo-' ventricular groove, #, and 

 two comparatively large twigs pass down the auricular septum 

 to the region of the valve, i, and there enter a collection of nerve 

 cells which, with other cells lying in the groove, constitute the 

 auricula-ventricular or Bidder's ganglion. From that gan- 

 glion nerves are continued to the w r all of the ventricle, and 

 near its base have nerve-cells mixed with them. A few 

 nerve-cells are also found among the fibres running down the 

 auricular septum : in the apex of the ventricle, however, and 

 in the bulb there are no ganglion-cells, though nerve-fibres 

 are present. We find then a considerable collection of nerve- 

 cells in the walls of the venous sinus, a few cells in the au- 

 ricular septum, a considerable collection at the junction of 

 atrium with ventricle, and a few scattered cells in the neigh- 

 boring portions of the ventricle. The cells of the ganglion 

 of Eemak and some of those in the septum belong to a type 

 differing somewhat from those hitherto described. Each is 

 pear-shaped, and has a conspicuous nucleus with a nucle- 

 olus; from the narrow end of the cell proceeds a branch 

 which ultimately becomes the axis cylinder of a medullated 

 nerve-fibre. Another branch arises by two. or more roots 

 which coil spirally around the straight branch, and finally 

 unite and proceed as a non-medullated fibre. Most of the 

 remaining nerve-cells of the frog's heart are spindle-shaped, 

 and receive a nerve-fibre atrone end and give one off at the 

 other. They are known as bipolar cells. The cardiac nerve, 

 N, Fig. 99, contains both gray and medullated fibres, the 

 latter coming entirely or almost entirely from its vagus root; 

 as the fibres passing on from the sinus ganglion to the gan- 

 glion of Bidder contain very few medullated fibres, it is prob- 

 able that many of the vagus fibres end in the pear-shaped 



