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INTRODUCTION TO NEUROLOGY 



tion or other biological agencies because this is an adaptation useful to the 

 organism. No separate neurons would be required for the transmission and 

 analysis of painful stimuli in their simpler forms. A peripheral neuron, say, 

 of the pressure sense, if excited by the optimum stimulus will transmit the 

 appropriate nervous impulse to the tactile centers of the thalamus and cere- 

 bral cortex. But the peripheral sensory neurons branch widely within the 

 spinal cord and there effect very diverse types of connection (see Fig. 61, p. 

 134) ; and supernormal or maximal stimulation of the end-organ may excite 

 so strong a nervous discharge as to overflow the tactile pathway in the spinal 



To the thalamus 



Fasciculus proprius 



Spinal lemniscus 



Spinal nerve 



Fig. 117. Diagram of the pathways of painful sensibility in the spinal 

 cord. The spinal lemniscus is the dominant path in the human body, and 

 the fasciculus proprius is the dominant path in other mammals. 



cord by overcoming the synaptic resistance of certain other collateral path- 

 ways with a higher threshold than those of the tactile path, thus exciting to 

 function the pathway for painful sensibility with its own central connection 

 in the thalamus (Fig. 118, A). 



In the course of the further differentiation of the cutaneous receptors, 

 the peripheral fiber of the sensory neuron may branch and effect connection 

 with two types of sense organs, one organ (a tactile spot) with a low thresh- 

 old for pressure stimuli whose nervous impulses are so attuned as to dis- 



