364 THE ANIMALS AND MAN 



cortex of the brain where the center of sight, or visual center, 

 is located. The cells in the visual center transmit the im- 

 pulse to its dendrites. These dendrites are in contact with 

 dendrites from the auditory centers, the intellectual center, 

 the speech centers, etc. Any of these dendrites or all of 

 them may be stimulated. The stimulation might thus 

 result in speech, or in motion with the hands or feet, or it 

 might produce a thought (fig. 182). 



The manner in which nervous impulses originate is 

 imknown, but it is believed to be due to the character of 

 the metabolism of the nerve cells. 



Classification of nerve fibers. — A nerve fiber is either 

 sensory (afferent) or motor (efferent) or commisural. Sen- 

 sory fibers carry impulses from the cell endings in the tissues 

 to the cell bodies within the central nervous system. Motor 

 fibers carry impulses away from the cell body within the 

 central nervous system to the cell endings in the tissues. 

 Thus in fig. 181 the motor root contains a mass of 

 efferent fibers, while the sensory root contains a mass 

 of afferent fibers. A stimulus at i produces a motion at 5. 

 Commisural fibers connect cells within the central nervous 

 system. 



A reflex action. — Such an action as that pictured in 

 fig. 181 is known as a reflex action, because it takes place 

 without the exercise of the will. In this case no impulse 

 passes up through the spinal cord to the brain calling into 

 play the intellectual centers, or consciousness. The spinal 

 cord is the center of most, though not of all, reflex action. 

 Most reflexes are more complex than the one illustrated, 

 for a sensory impulse may stimulate several motor nerves 

 of the cord and a complicated set of actions may result. 



Excitation and inhibition of reflexes. — Certain nerve 

 fibres carry impulses that excite action, while others carry 

 impulses that inhibit or keep from action the muscles or 

 glands to which they are distributed. We do not know 



