ANATOMY AND PHYSIOLOGY 



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however, ganglion cells from which processes may pass directly into 

 nerve fibrillae. 



A nerve-fiber, in an insect, consists of an axis-cylinder, composed of 



D ^ 



FIG. 112. Successive stages in the concentration of the central nervous system of Diptera. 

 A, Chironomns; B, Empis; C, Tabanus; D, Sarcophaga. After BRANDT. 



fibrillae, and an enveloping membrane, or sheath. The axis-cylinder is 

 the transmitting portion and the ganglia are the trophic centers, i. e., 

 they regulate nutrition. A nerve is always either sensory, transmitting 



FIG. 113. Nervous system of the head of a cockroach, a, antennal nerve; ag, anterior 

 lateral ganglion of sympathetic system; b, brain; d, salivary duct; /, frontal ganglion; h, 

 hypopharynx; , labrum; li, labium; m, mandibular nerve; mx, maxillary nerve; nl, nerve to 

 labrum; nli, nerve to labium; 0, optic nerve; oc, cesophageal commissure; oe, oesophagus; 

 Pg, posterior lateral ganglion of sympathetic system; r, recurrent nerve of sympathetic system; 

 s, subcesophageal ganglion. After HOFER. 



impulses inward from a sense organ ; or else motor, conveying stimuli from 



the central nervous system outward to muscles, glands, or other organs. 



Functions. The brain innervates the chief sensory organs (eyes and 



