THE NERVOUS SYSTEM. 165 
mits an order by an efferent nerve to the muscles of the 
hand to contract. If the former 
are cut, sensation is lost, but vol- 
untary motion remains; if the cc 
latter are cut, the animal loses ba 
i} 
all control over the muscles, al- 
though sensibility is perfect; if 
SB 
ATK 
both are cut, thé animal is said 
to be paralyzed. The nature of f 
nerve-force, and exactly how the f 
nerves terminate in the skin and /] ff) § 
muscles, are unsolved problems. Ag 
vw.» As to the velocity if 
WAZ: f : \ 
ya = or a nervous 1M- yy. 133. — Nervous System of a 
\ VA pulse we know it Mollusk, the Gasteropod Aplys- 
: A Sex ae 5 : qa: a, anterior ganglion; e¢, ce- 
SK 2 is far less than  phalic; J, lateral; g, abdominal. 
ogee a mae : Plane 
UISa that of electricity or light, and that it is 
lyas 
Fig. 134. — Nervous 
System ofa Cater- 
pillar (Sphinx li- 
gustri); the first 
is the cephalic, or 
head, ganglion. 
more rapid in warm-blooded than in cold- 
blooded animals, being nearly twice as fast 
in Man as in the Frog. 
Nervous matter in the form of cells 
doubtless exists in the very lowest ani- 
mals, although it is invisible under the 
most powerful microscopes." But a nerv- 
ous system of centres and nerves for keep- 
ing up a communication between different 
parts of the body is not required in such 
as have no distinct organs; we would look 
for it only in those possessing a well-de- 
fined muscular system. In the Star-fish 
we detect the first clear specimen of cells 
and fibres connected together to receive 
and convey impressions. It consists of a 
ring around the mouth, made of five gan- 
glia of equal size, with radiating nerves. The Mollusks 
