8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. JJ 



stance. Within it there are often more or less definite areas formed 

 by the finely-branching terminals of associated fibers. Such areas 

 are known as the neuropiles of the ganglion. When the fibers of a 

 neuropile are scattered and the limits of the region are not well de- 

 fined, the neuropile is said to be diffuse ; when the fibers constitute 

 a definite mass, the neuropile is called a glomerulus. In each body 

 ganglion there is a dorsal motor neuropile region, and a ventral sen- 

 sory neuropile region. 



The principal ganglionic nerves issue as paired lateral trunks 

 from the sides of each ganglion, but there is often also a posterior, 

 unpaired, median nerve, which may be continued from one ganglion 

 to that following. The fibers of the lateral nerve trunks separate 

 within the ganglion into dorsal and ventral bundles, or roots, in 

 which the fibers of the dorsal roots (fig. 3, MNv) are motor axons, 

 and those of the ventral roots {SNv) are sensory axons. This ar- 

 rangement is just the reverse of that which exists in the spinal 

 cord of vertebrates (fig 7B); but the main nerve centers of the 

 Arthropoda and the Vertebrata are developed from opposite surfaces 

 of the body — they are alike, therefore, in that the sensory roots in 

 each are those nearest the exterior. 



In each body ganglion there are five principal elements : ( i ) the 

 cytons and roots of the motor fibers of the lateral nerves; (2) the 

 roots of the sensory fibers of the lateral nerves; (3) the cytons 

 and fibers of the intraganglionic connectives ; (4) the cytons and 

 fibers of the longitudinal commissural nerves; (5) the cytons and 

 roots of the median nerve. 



The nerve cytons, or nerve cell bodies, of the motor fibers of the 

 lateral nerves, according to the diagram of Zawarzin (fig. 3, a, b), 

 lie in the dorso-lateral parts of the ganglion. Each cell gives off a 

 nerve process which soon divides into two branches. One branch, 

 the collateral, goes inward and ends with a fine aborization in the 

 motor neuropile of the ganglion ; the other, the axon, turns outward 

 in the dorsal root of a lateral nerve to become a peripheral motor 

 fiber (MNv). The sensory fibers (SNv), coming in from the pe- 

 riphery, separate from the motor root fibers of the nerve trunk, and 

 enter the ganglion through the ventral nerve root. Some of the 

 sensory fibers end in the ventral sensory neuropile of the ganglion ; 

 others merely give off branching collaterals into the sensory neuro- 

 pile, while the main axon proceeds forward through a longitudinal 

 commissure to the sensory neuropile of some anterior ganglion. 

 According to Zawarzin (1924), all the sensory fibers of the ab- 

 dominal ganglia of a dragonfly larva (Aeschna) end in ganglia an- 



