ANATOMY ANID PHYSIOLOGY 93 
(Punktsubstanz) derived from the processes of the cortical 
ganglion cells and serving as the place of origin of nerve fibril- 
le. There are, however, ganglion cells from which processes 
may pass directly into nerve fibrillee. 
A nerve, in an insect, consists of an axis-cylinder, composed 
of fibrillz, and an enveloping membrane, or neurilemma. ‘The 
axis-cylinder is the transmitting portion and the gangha are 
Transverse section of an abdominal ganglion of a caterpillar. a, axis-cylinder; g, 
ganglion cells; m, 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 antennz) 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 1f 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 subcesophageal ganglion controls the mouth parts, co- 
ordinating their movements as well as some of the bodily 
movements. 
