THE NERVOUS SYSTEM. 



167 



each having its distinct office: one carries impressions re- 

 ceived from the external world to the gray centres, and 

 hence 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 the} 7 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 Fio. 133._Nervous System of Star- 



imnrpsm'on to the centre the fish: Diagram -r, nervous r i ng 



impression 10 iae centre uie around mouth . w , radial nerves to 



spinal COrd, which immediately each arm, ending in the eye. 



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 

 warm-blooded than in cold-blood- 

 ed animals, being faster in Man 

 than in the Frog. In the latter 

 it averages about 85 feet per second, in the former over 

 100 feet. 



Fig. 134. — Nervons System of a 

 Mollusk (the Gasteropod Aplys- 

 ia) : a, anterior ganglion ; c, ce- 

 phalic ; I, lateral ; g, abdominal. 



