THE SPINAL CORD AS A REFLEX CENTRE 379 



the latter impulses in determining the myotatic irritability of the 

 muscles is especially marked in man, where total transverse lesion 

 of the upper part of the spinal cord often abolishes permanently 

 the tone of the muscles innervated from the lower portion of the 

 cord, and especially the knee-jerk. How far this absence of tone 

 is due to collateral changes in the cord has not yet been determined. 

 In animals complete transverse section of the cord of the cervical 

 region is followed by increase of the knee-jerk, which in the rabbit may 

 be elicited within a quarter of an hour after the section has been carried 



L 3 



ANT? CRUR . N. 



S.2. 



Fro. 169. Diagram to show muscles and nerves concerned in Sherrington's 

 experiment on the reciprocal innervation of antagonistic muscles. 



L3, lA, L5, third, fourth, and fifth lumbar roots ; si, s2, first and second 



sacral roots. 



out. In the increased myotatic irritability observed after removal 

 of the cerebral cortex, or after degeneration of the pyramidal tracts 

 coming from the motor cortex, a single tap on the patellar tendon 

 may evoke a series of contractions of the extensor muscles of the 

 thigh, giving rise to what is known as knee clonus. In the same way 

 forcible flexion of the ankle causes a series of rhythmic contractions 

 of the calf muscles (ankle clonus), varying in rhythm from six to 

 ten per second. The heightened tone of the muscles under these 

 conditions, and the ease with which any slight increase in their 

 tension gives rise to clonic contractions, cause such patients to have 

 a peculiar dancing gait, characteristic of pyramidal degeneration, 

 and known as the ' spastic ' gait ; it is generally associated with a 

 certain loss of voluntary control of the movements of the limbs, so 

 that the whole complex of symptoms is called ' spastic paraplegia.' 



