STRUCTURE AND FUNCTIONS OF THE CEREBRUM 179 



another the head. They have shown incidentally, also, that the 

 cerebral cortex is not painfully sensitive to direct stimulation. 

 The men whose brains were excited electrically in the observa- 

 tions cited above were conscious throughout the procedure and 

 reported no sensations of pain or discomfort at any stage. 



Cortical Reflex Paths. The various sensory areas with their 

 afferent nerve-paths afford means whereby impulses may enter 

 the cerebrum from the different receptors; the motor areas, one 

 in each hemisphere, with their efferent paths, provide for the 

 passage of impulses from the cerebrum to the motor organs of the 

 Body; the abundant equipment of association fibers within the 

 cerebrum makes possible the passage of impulses across from 

 sensory areas to motor areas. We can picture, then, reflex arcs 

 involving the cerebrum. Such arcs are necessarily complex, in- 

 volving many more neurons than dp the simple spinal cord reflex 

 arcs already described. In a previous paragraph (p. 158) we saw 

 that the simplest reflex arc through the cord involves at least two 

 neurons, one sensory, 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 cuneate 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 neuron 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, conveying the impulse from the 



