200 THE HEART. 



IV . THE CARDIAC NERVES AND GANGLIA. 



Limulus gives us the most detailed available picture of the structure and 

 relations of the cardiac nerves in invertebrates. 



We recognize five longitudinal cardiac nerves extending either over the sur- 

 face of the heart, or close to it; and seven or eight pairs of segmental or trans- 

 verse ones, which connect the longitudinal nerves with the vagus or branchial 

 neuromeres. 



The Median Cord or Ganglion. This is the primary nerve center for the 

 heart. It is a median cord of ganglion cells, readily visible to the naked eye, 

 lying on the haemal surface of the heart and extending from one end of the 

 heart to the other. (Fig. 115, m.c.n.} 



It arises at an early embryonic period from a thickening of the overlying 

 ectoderm... It probably -extended, in primitive arthropods, the whole length of 

 the body. In the forms I have studied (Acilius, Limulus, and scorpion), it extends, 

 during the embryonic period, over a longer territory than the heart and appears 

 to stand in the same relation to the haemal surface of the body that the middle 

 cord does to the neural surface. The median cord, therefore, belongs primarily 

 to the overlying ectoderm. It lies on the outer surface of the myocardium, and 

 is not at any point actually imbedded in it. 



All the ganglion cells of the heart lie in the median cord, or at the roots of 

 the strands that arise from it. 



The cardiac plexus is an irregular meshwork of nerve fibers, arising from 

 the median cord and spreading over the haemal surface of the heart. The strands 

 diminish in caliber and finally form slender bundles that penetrate the walls 

 of the heart, where most of them appear to end in minute end plates, on the surface 

 of the muscle strands. 



The Lateral Cardiacs. Many of the larger strands of the plexus unite on the 

 sides of the heart to form distinct lateral nerves, easily visible to the naked eye, 

 l.c.n. 



The median cord, the plexus, and the lateral nerves stand out with great dis- 

 tinctness when treated with methylene blue, and they may be easily studied under 

 high powers, in preparations of the whole heart of either the young or the adult 

 animal. The lateral cardiacs never contain ganglion cells. 



The pericardial nerves extend lengthwise on the side walls of the peri- 

 cardial chamber, breaking up, at either end, into minute fibers, the terminations 

 of which could not be certainly ascertained, p.ii. 



The Segmental Cardiacs. There are seven or eight pairs of segmental 

 cardiac nerves, s.c.n. 7 They arise from the seventh to the thirteenth haemal 



nerves inclusive, in close connection with the rami communica'ntes of the hypo- 

 branchial nerves. (Fig. 59.) They extend to the haemal side of the body, giving 

 off numerous small branches to the neighboring muscles and integument. The 

 five branchial ones turn inward, giving off rami communicantes to the peri- 



