REFLEX ACTION OF THE SPIXAL CORD. 303 



" On the one hand, it is clear that an influence, inde- 

 pendent of the will, occasionally throws voluntary muscles 

 into action, as appears in tetanus and other spasmodic dis- 

 orders; and is shown remarkably in the physiological ex- 

 periment of irritating the skin on the lower extremities, after 

 the division of the spinal cord in the back, when the occur- 

 rence of action limited to the muscles of the inferior extremi- 

 ties, evinces that a connection exists, independently of the 

 will, between sentient surfaces and the action of voluntary 

 muscles. I have varied this experiment by dividing the 

 spinal cord at once in the neck and in the back, upon which 

 three unconnected nervous centres exist ; and the division 

 of the skin of either part (and especially at the soles of the 

 feet, in the two hinder portions) produces a convulsive action 

 of the muscles of that part alone. The same influence may, 

 then, possibly regulate the unconscious actions to which 

 these remarks relate." 1 



The experiments of Marshall Hall, published in 1832 

 and 1833, are familiar to every physiologist, as supplying 

 nearly all of the omissions of the observers just cited. The 

 points which he assumed to have experimentally demon- 

 strated by his researches are as follows : A decapitated ani- 

 mal, the only part of the cerebro-spinal axis which remains 

 being the spinal cord, will make no movements, if complete- 

 ly protected from all external impressions. An impression 

 made upon the sensory nerves of a decapitated animal is 

 reflected by the cord, through the motor nerves, to the mus- 

 cles, and gives rise to reflex movements. If the cord be 

 destroyed, no movements follow stimulation of the surface. 

 If the centripetal and the centrifugal nerves be divided, no 

 reflex movements can take place. Experiments upon de- 

 capitated animals accord with the results of observations 

 upon acephalous foetuses, and in cases of complete paraplegia 

 from injury to the cord. All of the involuntary movements 



1 MAYO, Anatomical and Physiological Commentaries, Number II., July, 1823, 

 London, 1823, p. 17. 

 120 



