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560 NERVOUS SYSTEM 



Reflex Action of the Spinal Cord. Simple reflex action involves the 

 existence of an afferent (sensory) nerve, a collection of nerve-cells, and 

 an efferent (motor) nerve, the nerves being connected with the nerve- 

 cells. In a decapitated animal, not only are the movements independent 

 of sensation and volition, but no movements occur if the sensory nerves 

 are protected from any kind of impression or stimulation (Marshall Hall, 

 1832 and 1833). If the cord is destroyed, however, no movements 

 follow stimulation of the surface ; and if either the afferent or the 

 efferent nerves are divided, no reflex movements can take place. Experi- 

 ments on decapitated animals are in accord with the results of observa- 

 tions on acephalous foetuses and cases of complete paraplegia from 

 injury to the cord. 



In the simplest form of a reflex movement, the muscular contraction 

 is confined to the muscle or muscles which correspond, in their nervous 

 supply, to the afferent nerve stimulated ; but when the stimulus is 

 sufficiently powerful or when the cord is in a condition of exaggerated 

 excitability, the impression is disseminated throughout the gray matter, 

 and the entire muscular system may be thrown into action. With feebler 

 stimulation, one side only of the muscular system may respond. When 

 the reaction extends to the opposite side, it is called crossed reflex. The 

 extension of a stimulus conveyed by a single afferent nerve throughout 

 the cord is called irradiation. 



When a feeble stimulus applied to an afferent nerve is repeated 

 frequently and at short intervals, general muscular movements are pro- 

 duced. This follows stimuli applied three Jimes in a second, and the 

 effect is increased up to sixteen shocks in a second, but not beyond this 

 number (Rosenthal). 



In studying the paths of conduction in the cord, it has been seen 

 that sensory conduction takes place through the gray matter and pos- 

 sibly through the columns of Goll, that motor impulses are conducted 

 by the direct and the crossed pyramidal tracts, and that the columns of 

 Burdach probably are connected with muscular coordination. The fibres 

 of the cord that are specially concerned in reflex action probably are in 

 the anterior ground columns and the lateral bundles. 



It is well known that the reflex excitability of the cord is exaggerated 

 by removal of the encephalon. According to Setschenow, certain parts 

 in the encephalon, particularly the optic lobes in frogs, exert an inhibi- 

 tory influence over the reflex acts of the cord, and as a consequence, 

 reflex phenomena are more marked when this influence has been 

 removed. 



Certain poisons, especially strychnin, have a remarkable influence 

 over reflex excitability. In a frog decapitated and poisoned with 



