102 PERIPHERAL NERVES AND GANGLIA. 



an immense bilobed ganglion (ganglion nodosum) composed of ganglion cells and 

 concentric lamminse of medullary substance (g.nd). 



It is united with its mate by two distinct bridges of nerve tissue that lie some 

 distance above the surface of the neuromeres they thus form an imperfect roof to 

 a deep, narrow canal between the two ganglia and the median sides of the under- 

 lying neuromeres (Fig. 40). The anterior ends of the ganglia may be traced in 

 transverse sections a long distance forward, as two great longitudinal fasciculae, 

 just below the neural surface of the thoracic neuromeres. (Fig. 69, v.g.l. and 

 g.t.r.) 



The ganglion on the second root (ganglion fusiforme) is smaller, spindle- 

 shaped, and as near as can be determined, appears to belong to the third neuromere. 

 The third root is small, fibrous, and without any ganglionic enlargement. 



The Haemal Nerves. The two haemal nerves of the first neuromere remain 

 separate, as in a typical thoracic neuromere. In each of the three following 

 neuromeres, they unite to form a single nerve, each with a double root. (Fig. 42, 

 h.n. 1 ' 4 .) A short distance from the brain all five haemal nerves form a compact 

 bundle that extends backward through the occipital foramen of the cartilaginous 

 cranium. (Figs. 71 and 217.) The nerves to the third and fourth neuromeres, 

 h.n. 3 and h.n. 4 , some distance from the brain, fuse to form a single nerve supplying 

 the first and second lung books and the ventral surface of the body (Fig. 71). 

 On its way to these organs, it passes over the ventral surface of the liver, to which 

 it possibly gives branches. The anterior haemal nerve of the first vagus neuromere, 

 h.n. 1 runs close to the coxal gland, and dividing into numerous branches, is lost 

 on the surface of a thick, peritoneum-like membrane. The posterior nerve, 

 h.n. 2 extends along the arthroideal membrane, supplying numerous sense organs 

 on the lateral and the haemal surface of the abdomen. The fourth vagus nerve, 

 h. n. 4 supplies the skin and the longitudinal muscles on the ventral surface of the 

 abdomen. 



IV. NERVES OF THE BRANCHIENCEPHALON. 



The branchial neuromeres differ from those of the brain in that they remain 

 separate through life. Their nerves are noteworthy for their association with the 

 respiratory muscles, the heart, and the intestine. 



Limulus. 



The Branchial Nerves. In Limulus, the branchial, or neural nerves, contain 

 both motor and sensory fibers. They arise from large ganglia on the posterior 

 neural surface of the neuromeres ; on entering the gills they divide into three branches. 

 (Fig. 82.) The external branch, eb.n. supplies the abductor muscles and the skin 

 on the anterior lateral surface of the appendage. The median branch g.n. 

 supplies the adductor muscles and the gill books. The internal branch i.bn. 

 upplies the skin and muscles in the terminal portion of the appendage. 



