ON THE FUNCTIONAL ACTIVITY OF NERVE CELLS. 517 



would raise is the following : Is this increase of resistance in the neural 

 conduction due to change in the mutilated afferent nerve cell — e.g., retrac- 

 tion of its cell-processes withdrawing them from their normal apposition 

 against the motor cell— or is it due to change in the motor cell or its pro- 

 cesses — e.g., retraction of its dendrites. 



The subjoined observations throw, I think,- some light on this point. 



1 . In the monkey and cat after spinal transection at the top of the cervical 

 region flexion of the hind limb can be elicited by stimulation of the skin 

 of the fore limb of the same side— e.g-., forepaw. Section of the afferent 

 spinal roots of the hind limb, sets aside this reaction. 



2. Similarly, in monkey and cat excitation of the pinna of the ear 

 elicits flexion in the hind limb of the same side of the body, but section 

 of the afferent roots of the nerves of the hind limb sets the reaction aside. 



3. When, after transection on the cerebral side of the pons in cat, the 

 condition of extensor rigidity which I have described elsewhere (Proc. 

 R. S. 1896), and termed ' decerebrate rigidity,' has set in, severance of the 

 afferent roots of the nerves of the limbs immediately abolishes this rigidity. 



It would appear from these observations, therefore, that the severance 

 of the afferent roots exercises an effect upon the motor nerve cell itself. 

 The effect is such as to cause some change in the motor nerve cell that 

 makes it less accessible not only to the afferent fibres which have been 

 ruptured from their own parent nerve cells, but also to various other 

 afferent fibres. 



An objection may be raised against this conclusion on the ground that 

 the mere operation of section of a number of afferent nevxe roots involves 

 necessarily the opening of the vertebral canal and the laying bare of a 

 portion of spinal cord, and that that may of itself depress all the functions 

 of all the elements in the spinal cord thus treated. This consideration 

 appears to me a very valid one, and I believe that as a necessity the 

 operative procedure does tend to depress the activities of the cord where it 

 is exposed. But the following observations, I think, show that the depres- 

 sion of conduction cannot be explained in the above cases by the mere 

 influence of the operation. 



1. The excitation of the pyramid tract becomes rather more effective 

 after the section of the afferent roots than it was before ; that is, the 

 connection between the endings of the pyramid tract fibres and the motor 

 cells of the ventral horn are more easy and patent than previously ; that is, 

 an exaltation of function in&tead of a depression has occurred in this 

 nexus. 



2. In the monkey and cat, after spinal transection at the top of the 

 cervical region, although it is easy to obtain flexion of the hind-limb by 

 excitation of the fore-limb of the same side of the body, or of the pinna of 

 the ear of the same side as the hind-limb in which the reflex movement 

 occurs, it is extremely difficult — in my experience often altogether impos- 

 sible — to obtain from one fore-limb or pinna any movement of the hind 

 limb of the crossed side. After section of the afferent roots of the nerves of 

 a hind-limb it becomes comparati\ ely easy, however, to elicit in the crossed 

 hind-limb movements by excitation of fore-limb and pinna of the same 

 side of the body as that upon which the afferent roots have been severed. 

 But to cut the afferent roots both sides of the spinal cord were in the 

 preliminary operation laid bare. 



It seems, therefore, that severance of the afferent fibres to a limb, pro- 

 bably by interrupting a normal continuous conduction along those fibres, 



