REFLEX ACTION OF THE CORD. 275 



cord be now irritated, convulsive movements follow in the muscles 

 below and above, resulting from the indirect artificial stimulation of 

 the motorial tracts and motor nerves proceeding from them, and not 

 from a direct irritation of those tracts, the effects of which would be 

 limited, as in the case of a pure motor nerve, to the muscles supplied 

 from the stimulated portion of the cord. But, furthermore, move- 

 ments may be excited in those muscles, by the application of a stimulus 

 to some distant and excitable part of the skin of the animal ; the stim- 

 ulus employed, first excites the extremity or trunk of an afferent or 

 so-called sensory nerve, the effect of the impression so produced pass- 

 ing up to the gray nervous centre, the cord, and thence being, as it is 

 specially termed, reflected on to certain efferent, or so-called motor, 

 fibres, and so reaching the muscles which are excited to contract. 

 This is the mechanism of all reflex movements. All require for their 

 execution, an afferent nerve, an efferent nerve, and an interposed 

 gray nervous reflecting centre, or centre of reflection. The stimulus 

 excites the afferent nerve, this the reflex centre, and this again the 

 efferent nerve ; hence the term excito-motor, or excito-motory, applied 

 to the reflex phenomena and acts. They are also named automatic. 



The experimental illustrations just given, in the. cases of decapitated 

 animals, or of animals the spinal cord of which has been divided, show 

 the independence of these acts, of the cerebrum, or of any cerebral 

 interference; they prove accordingly, that the spinal cord is, in regard 

 to them, an independent centre of nervous action. These movements 

 are strictly involuntary, and they may occur quite independently of 

 sensation and consciousness; they are not irregular or convulsive 

 movements, such as follow the pinching or irritating a motor nerve, 

 or the motor columns of the cord ; but they are definite and regulated 

 movements, depending on the distribution of the afferent nerve, to the 

 ends of which the stimulus is applied, and on the particular efferent 

 fibres, upon which the effects of this stimulus are reflected. 



These reflex movements are even more extensive and powerful, 

 when the spinal cord is separated from the brain, or seat of volition, 

 than when the cord and medulla are still connected with it. Accord- 

 ing to some, this is owing to the loss of a controlling power exercised 

 by the brain ; but according to another view, it is rather due to the 

 fact, that when the cord is severed, the whole force of the excitation 

 is necessarily thrown upon the ganglionic centres of the cord alone. 

 Reflex movements are more easily excited by irritation applied to the 

 free extremities of the afferent nerves, as to the skin of a frog, than 

 by stimuli, even of a stronger kind, applied to the trunks of those 

 nerves, though the pain in the last case may be as great or greater. 

 This is another proof, that there is no necessary relation between sen- 

 sation and reflex action. So in cases of diminution of sensation, and 

 loss of voluntary motion, in the lower half of the body, the controlling 

 power of the brain being absent, the reflex action is increased, and slight 

 stimulation of the skin, unfelt by the person, produces more powerful 

 reflex movements, than the stronger stimuli of pinching and pricking, 

 which may be felt by him. The phenomena observed, after injury of 

 the cord, further prove, even in the human subject, that the convey- 



