REFLEX ACTION OF THE CORD. 277 



to move, he replied, "No, but you see my legs do." Such cases de- 

 monstrate perfectly, not only the involuntary character of the reflex 

 movements in question, but also prove that they may take place with- 

 out conscious sensation. The apparatus concerned in their production, 

 is exactly sirniliar to that of the movements excited by irritation of 

 the skin, in the hind limbs of the frog, the spinal cord of which has 

 been divided ; that is to say, it consists essentially of afferent or in- 

 cident nerves, of a reflex gray centre, and of efferent or motor nerves. 

 In certain experiments on the frog, the purposive character of these 

 movements, even where there can be no suspicion of volition or con- 

 sciousness, which must be one and indivisible, is well illustrated. 

 Acetic acid, which powerfully stimulates these animals, when applied 

 to the inner side of the knee joint, or to the side of the abdomen of a 

 decapitated frog, excites the animal to rub that portion of the skin 

 with the same foot; and, if now that foot be cut off, similar attempts 

 are then, more or less effectually, made with the opposite foot. 



Lastly, the necessity for the interposition of a reflex gray centre, 

 between the afferent and efferent nerves, is shown likewise by experi- 

 ments in animals, and by observation in cases of injury and disease in 

 man. For if the severed portion of the spinal cord in the frog be de- 

 stroyed, as by passing a wire down the spinal canal, stimulation of the 

 nerves of the skin will no longer produce reflex movements, although 

 the contractility of the muscles, and the excitability of their efferent 

 nerves, still remain active, as may be shown by pinching one of those 

 nerves, when the muscles, to which it is distributed, will immediately 

 contract. So, too, in cases of disease of the spinal cord, when the dis- 

 tal part, below the seat of any disintegrating injury or disease, becomes 

 itself softened, and so loses its vital properties, reflex movements can 

 no longer be excited in the lower limbs, by stimulating the skin. An 

 afferent nerve cannot therefore convey, or transmit, the effects of a 

 stimulus upon it, to an efferent nerve directly, but can only reflect 

 those effects indirectly, through interposed gray matter. From this, 

 it has been inferred that the nerve cells of the gray matter, are, in 

 some way, specially concerned in this office of reflection, but it is not 

 yet known how they act. The most prevalent opinion is, that a par- 

 ticular afferent fibre ends, as described in p. 53, in one of the processes 

 of a nerve cell, and that the efferent fibre arises from another process 

 of the same cell. Where the cells have more than one process, more 

 than one afferent or efferent fibre may, it is thought, be so connected 

 with it. Or a succession of nerve cells may be interposed between the 

 incident and the motor fibres, and so may extend or spread the effect 

 of the stimulus. Again, it is held by some, that perhaps the reflex 

 office of the gray matter, is effected by the mere proximity of the nerve- 

 fibres passing through it, between or amongst the nerve cells, without 

 there being any direct connection between them. 



A further question arises as to whether there are special afferent 

 fibres concerned in conveying a reflex stimulus, different from those 

 afferent fibres destined to convey the sensory impressions. It is cer- 

 tain that, if this be so, no anatomical difference between the several 

 afferent fibres in the posterior root of a spinal nerve can be detected. 



