THE NERVOUS SYSTEM. 
167 
fish: Diagram — r, nervous ring 
around mouth ; n, radial nerves to 
each arm, ending in the eye. 
each having its distinct office: one carries impressions re¬ 
ceived from the external world to the gray centres, and 
lienee is called an afferent, or sen¬ 
sory , nerve; the other conducts 
an influence generated in the 
centre to the muscles, in obedi¬ 
ence to which they contract, and 
hence it is called an efferent , or 
motor , nerve. Thus, when the 
finger is pricked with a pin, af¬ 
ferent nerve-fibres Convey the F ig. 133.-Nervous System of Star- 
impression to the centre — the 
spinal cord, which immediately 
transmits an order by efferent fibres to the muscles of the 
hand to contract. If the former are cut, sensation is lost, 
but voluntary motion remains ; if the latter are cut, the 
animal loses all control over the muscles, although sensi¬ 
bility is perfect; if both are cut, the animal is said to be 
paralyzed. The nerve-fibres are 
connected with nerve-cells in the 
central organs, and at the outer 
ends are connected with the mus¬ 
cular fibres, or with various sen¬ 
sory end-organs in the skin or 
other parts of the body. The 
nature of nerve - force is not 
known. As to the velocity of a 
nervous impulse, we know it is 
far less than that of electricity or 
light, and that it is more rapid in 
ifm. 134. — Nervous System of a WMBl-blooded than in Roltl-blood- 
Moiiusk (the Gasteropod Apiys - ec [ an i ma ] s being faster in Man 
phaiic; i, lateral; g, abdominal, than in the Frog. In the latter 
it averages about 85 feet per second, in the former over 
100 feet. 
