150 THE HUMAN BODY 



three neurons, one sensory, one association, and one motor. If 

 we trace a reflex arc involving the cortex from a receptor in the 

 skin of the right hand, for example, to a retractor muscle of the 

 right arm, we find in it at least five neurons and possibly many 

 more. The five which are necessarily included are: 1, the sensory 

 neuron which we suppose extends all the way from the receptor 

 into the cord and up the dorsal column to a termination in the cu- 

 neate or gracile nucleus; 2, a neuron of the fillet tract, having its 

 cell-body in the cuneate or gracile nucleus, and its axon extending 

 through the medulla and midbrain and the white matter of the 

 cerebrum, crossing the mid-line in the "sensory decussation " 

 of the fillet, and terminating in synaptic connection with a neuron 

 of the body sense area in the left cerebral hemisphere; 3, the neu- 

 ron just mentioned, having its cell-body in the body sense area 

 and an axon which passes by way of the cerebral white matter to 

 the motor area; 4, a pyramidal neuron of the motor area whose 

 dendrites receive the impulse from the body sense neuron (3), 

 and whose axon forms part of the pyramidal tract, crossing back 

 to the right side of the body in the decussation of the pyramids, 

 and terminating in synaptic connection with the cell-body of a 

 motor neuron in the ventral horn of gray matter of the cord ; 5, the 

 motor neuron which forms the last link in the reflex chain, con- 

 veying the impulse from the pyramidal neuron to the muscle. It 

 is doubtful whether any cortical reflex arcs are actually composed 

 of as few neurons as five; probably the simplest ones contain sev- 

 eral additional association neurons within the cerebrum. 



Cortical Reflexes Compared with Spinal Reflexes. As an ex- 

 ample of a simple spinal reflex was cited the involuntary with- 

 drawal of the hand from accidental contact with a hot body. 

 To illustrate a simple cortical reflex suppose that my finger rests 

 upon the terminals of an apparatus for generating electric shocks; 

 I am told that when I feel the shock I must withdraw my hand. 

 The shock may be so feeble as to be barely perceptible. Under 

 such circumstances the withdrawal must be voluntary and the re- 

 sponse, therefore, must involve the cerebrum. The chief objective 

 difference between voluntary withdrawal of the hand in response 

 to feeble stimulation, and its involuntary retraction in response 

 to strongly painful stimulation is that the former reaction requires 

 a noticeably longer time than does the latter. The only simple 



