FUNCTIONS OF THE SPINAL CORD 90! 



special, or, as it is termed, an ' adequate ' stimulus, so that it is 

 easily affected by this, and with difficulty or not at all by other 

 modes of stimulation. Thus, light is the adequate stimulus of the 

 end-organ of the optic nerve, heat that of the end-organs of the 

 nerves by which we perceive the sensation of warmth, mechanical 

 pressure that of the nerves by which we perceive the sensation of 

 pressure. Other kinds of stimuli are either entirely inactive or much 

 less effective in evoking the particular sensory response. There is 

 every reason to believe that the receptor in the reflex arc occupies -the 

 same position in regard to adequate stimuli as it does when it func- 

 tions as a sense-organ. 



$herrington has shown that the different kinds of nerve-endings 

 in one and the same area of the skin (in the dog) must be assumed to 

 possess totally different spinal connections, since the movements 

 elicited by stimuli suitable for one form of nerve-ending are quite 

 different from those elicited by stimuli suitable for another. 



The ' extensor-thrust ' is a reflex obtained in the hind-leg of the 

 dog, and characterized by a brief, strong extension at the hip, knee, 

 and ankle. . It is only elicited by a certain kind of mechanical stimu- 

 lation, best in the spinal dog i.e., in a dog whose brain has been 

 destroyed or severed from the cord some time before by pushing the 

 tip of the finger between the plantar cushion and the pads of the toes. 

 The stimulus is similar to that which normally liberates the reaction 

 namely, the pressure of the ground on the sole of the foot during 

 locomotion. The reflex cannot be obtained by electrical stimula- 

 tion or by any kind of direct stimulation of afferent nerve-trunks. 

 The same is true of the pinna-reflex in the cat i.e., the backward 

 crumpling of the ear elicited by squeezing or tickling its tip. The 

 scratch-reflex,* a scratching movement of the hind-foot, is much 

 more easily elicited in the spinal dog by mechanical stimulation 

 (rubbing, tickling, or tapping) applied to the skin of the back behind 

 the shoulder than by electrical stimulation, which often fails to 

 evoke it at all. The puzzling fact that, according to surgical ex- 

 perience, many of the internal organs e.g., the ureters and bile- 

 ducts can be handled, cut, and sutured without pain, while the 

 passage of a renal calculus or a gall-stone may cause excruciating 

 agony, becomes explicable in view of the apparently slight difference 

 which sometimes distinguishes an adequate from an inadequate 

 stimulus. Thus Sherrington has shown that very distinct reflex 

 effects e.g., a rise of blood-pressure can be obtained by sudden 

 distension of the bile-duct by the injection of salt solution into its 

 lumen. Distension is here the adequate form of mechanical stimu- 

 lation, and it is the form induced by the passage of a calculus, while 

 nerve-cutting, although a mechanical stimulus, is not an adequate one. 



* The scratch-reflex is very easily obtained in. cats during resuscitation 

 after a period of cerebral anaemia. 



