THE NERVOUS SYSTEM 



377 



contract, and hence it is called an efferent, or motor, 



nerve. Thus, when the finger is pricked with a pin, 



afferent nerve fibers convey the impression to the center, 



the spinal cord, which immediately transmits an order 



by efferent fibers 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 sensibility is perfect; 



if both are cut, the animal is 



said to be paralyzed in the parts 



which these nerves control. 



The nerve fibers are connected 



with nerve cells in the central 



organs, and at the outer ends 



are connected with the muscular 



fibers, or with various sensory 



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 FlG - 33^- Nervous System of a 



* Mollusk (the Gastropod Aplysta) : 



impulse, We knOW it is far leSS a, anterior ganglion; c, cephalic; 



,1 ,1 i , . ., 1 . i , /, lateral ; g, abdominal. 



than that or electricity or light, 



and that it is more rapid in warm-blooded than in 

 cold-blooded animals, being faster in man than in the 

 frog. In the latter it averages about 85 feet per second, 

 the former over 100 feet. 



The very lowest animals, like the amoeba and In- 

 fusoria, have no nerves, although their protoplasm has 

 a general sensibility. The hydra has certain cells which 

 are, perhaps, partly nervous and partly muscular in 

 function. The jellyfish has a nervous system, consist- 

 ing of a network of threads and ganglia scattered all 

 over its disk. We should look for a definite system of 

 ganglia and nerves only in those animals which possess 



