PAIN AND PLEASURE 



253 



charge centrally at the first synapse into the tactile tract, and another organ 

 differently constructed (a pain spot) which generates nervous impulses so 

 attuned as to discharge centrally into the pain tract (Fig. 118, B). In a 

 still more highly elaborated system two separate peripheral neurons may be 

 present to serve these functions, which are distinct throughout (Fig. 118, C). 

 All three of these methods of pain transmission and analysis may be present 

 in the spinal nerves; but by whatever pathway the pain impulses reach the 

 spinal cord, in the human body those which are destined to excite conscious- 

 ness of pain as a localizable sensation are immediately filtered off from the 

 other sensory qualities with which they may be associated and assembled in 

 a pathway of their own, which remains distinct from this time forth. With- 

 in the spinal cord and brain stem these pain impulses, especially those result- 

 ing from supernormal stimulation, also effect short reflex connections with 

 the adjacent motor centers for quick avoiding reflexes, and these may not 

 be associated with the spinal lemniscus, but with the more diffuse pain path 

 in the fasciculus proprius. 



pain path 

 tactile path 

 sKin 



pom path 

 tactile path 



spinal lemniscus-^. 



a, pain 



b, touch 



spinal cord 



Fig. 118. Three diagrams to illustrate various ways in which the 

 nerves of painful sensibility may be associated with those of other sensory 

 functions. 



The terminus of the ascending pain tract is related within the 

 thalamus very differently from those of the pathways for tactile 

 and thermal sensitivity. The latter impulses are in part trans- 

 mitted to the motor centers of the thalamus for intrinsic thalamic 

 reflexes, but chiefly pass forward after a synapse in the thalamus 

 through the internal capsule to the somesthetic areas of the 

 cerebral cortex. Head is of the opinion that the painful im- 

 pulses do not reach the cortex at all in their simple elementary 

 form, but that the painful sensations are essentially thalamic. 



Lesions of the lateral and ventral nuclei of the thalamus in- 

 volving the termini of the lemniscus, but leaving the geniculate 

 bodies and pulvinar and the medial and anterior nuclei intact, 

 result in the more or less complete loss of superficial sensation of 

 the opposite side of the body, with still more profound disturb- 



