80 SPINAL NERVE-ROOTS. 



more. Of this time, all but about a hundredth of a second 

 is occupied in the central process. 



It is often observed that the muscular effect produced 

 by the excitation of one afferent nerve is hindered or 

 delayed by excitation of another. This phenomenon is 

 called inhibition. This may be attributed either to the 

 counteraction of two centres, or to the counteraction of 

 two excitations in the same centre. 



Our knowledge of both kinds of reflex action is largely derived from the 

 observation of the phenomena exhibited by the body of the frog after the 

 animal has been killed by removing the brain. In preparations of this kind 

 it is seen (i) that definite series or groups of muscular actions adapted to 

 purposes, occur in response to excitation of particular spots of the cutaneous 

 surface ; (2) that slight excitations so applied, act by summation, i.e., do not 

 produce any effect until they have lasted for some time ; (3) that the excitation 

 of the central ends of nerve trunks produces irregular or convulsive reflexes, 

 the extent of which varies according to the intensity of the excitation ; (4) 

 that under the influence of strychnine, similar effects are produced, by ordinary 

 cutaneous stimuli ; (5) that by acting directly on the "convulsive centre" in 

 the medulla oblongata, or by faradization of the whole cord, general con- 

 vulsion is produced, similar to the partial effects above described (see 

 Practical Exercises). 



Functions of the Roots of the spinal nerves and of their 



Ganglia. 



It was discovered by Charles Bell, in 1 8 1 1, that mechanical 

 irritation of the anterior roots of the spinal nerves, pro- 

 duced convulsive movements of the muscles to which they 

 were distributed. More than ten years later, Magendie 

 discovered that excitation of the posterior roots produced 

 pain, and occasioned reflex contractions of the muscles, 

 and that these were prevented by section of the anterior 

 roots : subsequently he discovered that in mammals after 

 severance of an anterior root, excitation of the peripheral 

 end influences the cord through the trunk and posterior 

 root of the same nerve. In the frog the anterior roots 

 are exclusively afferent. 



Of the function of the ganglia of the posterior roots 



