PHYSIOLOGY OF THE INVERTEBRATE HEART. 153 



and the main nerve-trunks are confined to the dorsal side of the 

 heart. The large nerve-trunk on the dorso-median side of the 

 heart is in reality an elongated ganglion, being made up of nerve- 

 fibers and ganglion cells. This nerve-cord or ganglion extends 

 the whole length of the heart, but it is largest in the fourth, fifth 

 and sixth segments, tapering thence both in the anterior and the 

 posterior direction. There are relatively few ganglion cells in 

 the nerve-cord of the first and second segments. Besides this 

 elongated ganglion on the dorso-median side of the heart there 

 are two nerves running parallel with it and lateral to the ostia. 

 These are the lateral nerves. The nerves branch and anastomose 

 extensively, especially in the middle region of the heart. In 

 large specimens the nerves can be followed with the naked eye 

 up to the first, and posteriorly to the last segment, but they are 

 stoutest in the fourth to the sixth segments. There appears to 

 be no ganglion cells on the course of these nerves. A very com- 

 plex system of connectives extends between the median nerve- 

 cord and the lateral nerves. On the whole, one pair of these 

 connectives are given off from the median nerve-cord at the level 

 of each pair of ostia, the connectives being the largest in the 

 fourth, fifth and sixth segments. The connectives usually 

 branch extensively on the dorsal side of the heart before joining 

 the lateral nerves. The median nerve-cord, the lateral nerves as 

 well as the main connecting branches are large enough to be 

 easily made out with the naked eye in the living heart. 



The nervous complex on the dorsal side of the heart is partly 

 covered by the elastic connective tissue fibers, but these can be 

 removed without any injury to the nervous elements. Both the 

 nerve-cord and the nerves are, on the other hand, separated from 

 the heart-muscle by the basement membrane. This allows a 

 complete removal of the nervous complex without any injury to 

 the heart-muscle. 



The connections between this cardiac nervous complex and 

 the brain and the abdominal ganglia are shown in Figs. 24 and 

 25. These connections have been carefully worked out by 

 Patten and Redenbaugh, but these investigators failed to find any 

 connection between the haemal nerves given off from the posterior 

 end of the brain (7, 8) and the nerve-cord on the heart. These 



