L.3 



L.4. 



ANT? CRUR.N. 



THE SPINAL CORD AS A REFLEX CENTRE 335 



the tonic contraction of the extensor muscles. In the absence of such a 

 arrangement every movement of a joint, by stretching the antagonistic 

 muscles, would automatically increase their tone, and thus set up a resistance 

 to itself. The subject would thus be muscle- bound. 



Very great exaggeration of the tendon phenomenon is observed in cases 

 where the pyramidal tracts are degenerated, and indicates a heightened 

 reflex excitability of the lower spinal centres, perhaps reinforced by impulses 

 from the cerebellum. The importance of 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 *~- 5 

 cord, and especially the knee- 3. 1 

 jerk. How far this absence of 

 tone is due to collateral changes $.2 

 in the cord has not yet been 

 determined. In animals com- 

 plete 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 out. In the increased 



myotatic irritability observed concerned in" Sherrington's experiment on the 

 after removal of the Cerebral reciprocal innervation of antagonistic muscles. 



, . L3, L4, L5, third, fourth, and fifth lumbar roots; 



cortex, or after degeneration of slj s2> first and second sacral roots, 

 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 pyra- 

 midal degeneration, and known as the ' spastic ' gait ; it is generally associ- 

 ated with a certain loss of voluntary control of the movements of the limbs, 

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



The value of the tendon phenomena as a means of diagnosis has tended 

 to obscure their great importance in the normal individual. Every joint 

 is protected by inextensible ligaments and by muscles. A sudden strain on a 

 ligament either will have no effect, or will rupture some of its fibres and per- 

 haps injure the adjacent joint surfaces. An ordinary reflex contraction 



FIG. 168. Diagram to show muscles and nerves 



