164 COMPARATIVE ZOOLOGY. 
CHAPTER XVIII. 
THE NERVOUS SYSTEM. 
Nervous Matter exists in the form of cells, fibres, or 
tubes. In the cellular state it is grayish, and accumulated 
Vig. 131. —Nerve-cells from Human Brain: A, associ- 
ated with nerve-tubes and blood-vessels; B, multi- 
polar nucleated cells. 
in masses, called 
ganglia, or centres, 
which alone origi- 
nate nervous force; 
the fibrous and tu- 
bular kinds are gen- 
erally white, and 
arranged in bun- 
dles, called nerves, 
which serve only as 
conductors. Most 
nerves consist of 
white fibres, and 
go in pairs, each 
member haying a 
distinet office: one carries impressions received from the 
external world to the gray centres, and 
hence is called an afferent, or sensory, 
nerve; the other conducts an influence 
generated in the centre to the muscles, 
in obedience to which they, contract, 
and hence it is called an efferent, or 
motor, nerve. Thus, when the finger 
Fig. 132.—Nervous Sys- 
is pricked with a pin, an afferent nerve tem of aStar-fish: con- 
sisting of five ganglia, 
conveys the impression to the great cen- 9, around the mouth, 
which send to each ray 
tre—the brain, which immediately trans- a pair of nerves. 
