170 PATTEN AND REDENBAUGH. [Vol. XVI. 



The plexus upon the heart, and its connections with the 

 central nervous system through the segmental cardiac nerves 

 have never been described. The " deux filets delicats et tenus " 

 which Milne-Edwards describes, but does not represent in his 

 figures, we have not been able to find. 



(i) The Cardiac Plexus. — In a cross-section of the heart 

 (Text-figs. 12 and 14 ; PI. IX, Figs. 6 and 7) three large nerves 

 {l.c.n. and m.c.n.) are seen in the three angles. They lie upon 

 the outside of the heart between the longitudinal strands of 

 connective tissue. In Figs. 5, 8, 9, and 10 of the plates these 

 nerves are seen to better advantage, 



A median ganglionated nerve (m.cM.) traverses the heart 

 longitudinally upon the haemal side. Along the middle of its 

 course it is quite large, but dwindles down at the ends. Under 

 a low magnifying power (Pis. IX and X, Figs. 8 and 9) it ap- 

 pears as a large bundle of intertwining fibers intermingled with 

 masses of ganglion cells. Still higher magnification shows that 

 many of these ganglion cells are bipolar {g.c.y PI. X, Fig. 10). 



The lateral nerves {l.c.n.) of the heart are not ganglionated. 

 They traverse the sides of the heart just above the lateral angles 

 and communicate with the median nerve by an elaborate plexus, 

 which is richest upon the haemal sides of the heart. The 

 neural side of the heart seems to have very few nerves. 



The main branches of the cardiac plexus (PI. IX, Fig. 8) 

 arise from the median nerve uniformly in pairs opposite the ostia, 

 but the connections with the lateral nerves seem to be entirely 

 irregular. Some of the branches of the plexus approach very 

 closely the origins of the lateral arteries, but no nerves have 

 been observed running out onto them. 



(2) Segmefital Cardiac Nerves. — The segmental cardiac 

 nerves (Text-figs. 8, 9, 12, 14, 15, 16, and 18, Pis. VI-IX, 1-3, 5, 

 and 6, s.c.n.^'^) have been partially described in the foregoing 

 pages. Those of the five branchial neuromeres {s.c.n.^''^) are 

 most typical, and they will be considered first. They arise from 

 the haemal nerves {h.n.'^^^), in close connection with the recur- 

 rent branches of the lateral sympathetic, opposite the branchio- 

 thoracic muscles, and pass outside of these muscles to the 

 haemal side of the body, just anterior to the entapophyses 



