6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. // 



their root branches, receiving their stimuh secondarily from the 

 central roots of the afferent fibers, constitute what is in general known 

 as the motor nervous system, though the out-going stimuli induce 

 not only muscular contraction, but glandular and all other cell 

 activities. 



Besides the long nerves that enter or depart from the central 

 ganglia, there are shorter nerves, arising from cytons within the 

 ganglia, that are confined to the limits of one ganglion, and longer 

 nerves which go from one ganglion to another through the com- 

 missures. These nerves and their cytons constitute the association 

 system, consisting of connective fibers and commissural fibers, which 

 place different parts of the same ganglion, or parts of different 

 ganglia, in communication with one another. 



Finally, within the brain, there are two special regions with neu- 

 rons of their own, which receive association terminals from all 

 parts of the brain and the ventral nerve cord. These regions, known 

 on account of their shape as the mushroom bodies, constitute the 

 centers of the entire central nervous system. 



The motor nerves are in all cases outgrowths of cells located 

 in the nerve centers. The motor nerve cytons of insects lie either 

 within the ganglia of the brain and the ventral nerve cord, or in 

 the ganglia of the stomatogastric system. The peripheral sensory 

 nerves of most animals, on the other hand, arise from cells lying 

 outside the principal ganglionic centers, which cells either retain a 

 peripheral position or are withdrawn more or less deeply into the 

 body. 



In vertebrate animals, most of the sensory cytons are derived from 

 the neural crests of the embryo, ridges of ectodermal cells that are 

 separated from the ectoderm along the line of closure of the neural 

 tube, and which finally come to lie in the spinal ganglia. These 

 cells (fig. 7 B, SCy) send out axons which branch in one direction 

 (SNv) to the outlying parts of the body, and in the other to the 

 spinal cord, thus establishing sensory connections between the periph- 

 eral sense receptors of the body wall (BW) and the main nerve 

 centers. Only the nerves of the olfactory organs in vertebrates have 

 their origin in cells that remain permanently in the peripheral ecto- 

 derm. 



With insects, no neuroblastic cells are known that are analogous 

 to those of the spinal ganglia of vertebrates. The origin and growth 

 of the sensory nerves have not been traced in any insect, but all 

 investigators agree that the sensory nerves end in the central ganglia 

 in finely-branching terminal fibrils (fig. /A, SNv), which consti- 



