REFERRED PAIN 197 



medulla, and, when they give rise to referred pain, the pain 

 is always experienced on the same side of the body and never 

 spreads to the opposite side. 



The Viscero-motor Reflex. Pathological processes which 

 give rise to pain are frequently accompanied by muscular 

 contractions. This association is well seen in tuberculous 

 disease of the cervical vertebrae, in which the rigidity of the 

 muscles of the back of the neck is a striking feature. The 

 muscular contraction is quite involuntary, and it may be 

 explained as a result of the " viscero-motor reflex" of 

 Mackenzie. 



In order to account for the viscero-motor reflex, it is 

 necessary to assume that the afferent pathological impulses 

 "overflow" so as to affect the cells round which the fibres 

 of the pyramidal tract (p. 37) arborise. This "overflow" 

 stimulus produces precisely the same results as a stimulus 

 arising in the motor cortex, and it affects the motor nerves 

 which arise from the segment of the spinal medulla in which 

 the "focus of irritation " is situated. 



The viscero-motor reflex may be observed in the case of 

 structures innervated through the sympathetic system, and 

 many examples can be brought forward. The board-like con- 

 traction of the muscular abdominal wall in cases of acute 

 general peritonitis, the localised contraction of the upper part 

 of the right rectus abdominis in cholecystitis (p. 265), the 

 contraction of the cremaster in renal colic (p. 364), are all 

 instances of this condition. 



Particular examples of the viscero-sensory and viscero-motor 

 reflexes are detailed in the sections dealing with the individual 

 viscera (see Stomach, Ureter, Bladder, etc.). 



