202 THE HEART. 



cardial nerves; some of them unite with the median cardiac, in the region 

 of the last five pairs of ostia. (Figs. 115-117.) The segmental cardiacs of the 

 seventh and eighth, or vagus neuromeres, unite to form one large nerve which 

 anastomoses with the pericardial trunks, but neither it, nor the sixth, could be 

 traced directly to the median cardiac. 



********* 



The entire system of cardiac nerves probably represents a modification of a 

 primitive system of longitudinal and circular integumentary nerves distributed 

 to the skin, muscles, and other organs on the haemal surface of the body. With 

 the reduction of the primitive heart to a shorter, more compact organ, lying in the 

 posterior thoracic and branchial regions, there was a corresponding reduction 

 in the length of the several longitudinal nerves and in the number of pairs of seg- 

 mental cardiac nerves uniting the heart with the nerve cords. 



In some arthropods it is probable that there is some connection between the 

 cardiac and the stomodaeal nerves. (See Polici, Naples Mittheilung, 1908, and 

 other papers by the same author.) Such a connection, if it still exists in Limulus, 

 must be very minute, and can only be detected by a special application of the 

 methylene blue method. 



V. The Minute Structure of the Cardiac Ganglion. 



Nerve Cells. — We may recognize in the median cardiac nerve three different 

 kinds of ganglion cells: a. Small, multipolar cells that stain very quickly and 

 deeply in methylene blue, and that form a thick, irregular covering, several layers 

 deep, over the outer surface of the cord. (Fig. 116, gn.c'.) In exceptional cases, they 

 extend for some distance on to the larger, lateral branches of the plexus, where they 

 form irregular flakes or clusters; but I have never seen any isolated ganglion cells 

 on the lateral, or on the pericardial trunks, or on any of the smaller strands of 

 the cardiac plexus. I doubt very much whether any ganglion cells exist in the 

 heart outside the median cord, or the roots of the larger strands near where they 

 leave the cord. 



In the adult, these cells are about 32/_(. in diameter. They are pear- 

 shaped, the cell body giving rise to many fine dendrites which form dense felted 

 masses of varicose fibers. One can usually distinguish among them one long 

 fiber, extending inward, diagonally across the inner surface of the median cord, 

 and out of it, through one of the branches of the other side. 



These cells are very numerous in the heart segments belonging to the mid- 

 dle branchial neuromeres, where they form a thick and continuous but irregular 

 coating to the median cord. 



Toward the anterior end of the heart, the cord becomes much smaller, 

 and in the first three or four segments, these cells are either absent or reduced 

 to a few, scattering clusters, usually located opposite the ostia. h. The second 

 kind, Gn.c, consists of giant bipolar cells, which in the adult are about 140 x loofi. 



