ANATOMY AND PHYSIOLOGY 93 



(Punktsubstanz) derived from the processes of the cortical 

 ganglion cells and serving as the place of origin of nerve fibril- 

 lae. There are, however, ganglion cells from which processes 

 may pass directly into nerve fibrillge. 



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

 of fibrilke, and an enveloping membrane, or neurilemma. The 

 axis-cylinder is the transmitting portion and the ganglia are 



FIG. 115. 



r- 



Transverse section of an abdominal ganglion of a caterpillar, a, axis-cylinder; g, 

 ganglion cells; n, neurilemma; p, Punktsubstanz. 



the trophic centers, i. e., they regulate nutrition. A nerve is 

 always either sensory, transmitting 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 antennae) and converts the sensory stimuli that it 

 receives into motor stimuli, which effect co-ordinated muscular 

 or other movements in response to particular sensations from 

 the environment. The brain is the seat of the will, using the 

 term " will " in a loose sense ; it directs locomotor movements 

 of the legs and wings. An insect deprived of its brain cannot 

 go to its food, though it is able to eat if food be placed in con- 

 tact with the end-organs of taste, as those of the palpi ; further- 

 more, it walks or flies in an erratic manner, indicating a lack 

 of co-ordination of muscular action. 



The suboesophageal ganglion controls the mouth parts, co- 

 ordinating their movements as well as some of the bodily 

 movements. 



