THE NERVOUS SYSTEM. 



167 



a 



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 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 fio. las.-Nervous system of star- 



imnrpssinti- tn thp ppntrp thp *'*''= Diagram — r, nervous ring 



impression to me ceniie me around moatli ; n, radial nerves to 



spinal cord, which immediately eacli arm, ending in ttie 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 musclee, 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 tlie 

 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 

 ,f.G. 134. -Nervous System of a warm-blooded than in cold-blood- 

 Moiiusic (the Gasteropod Apiy>,- gd animals, being faster in Man 



ia) : a, anterior ganglion ; c, ce- 7 o 



phaiic; z, lateral ; j;, abdominal, than in the Frog. In the latter 

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

 100 feet. 



