336 



PHYSIOLOGY 



L.3 



L.4. 



U.S. 

 S.I. 



S.2. 



ANT? CRUR .N. 



Very great exaggeration of the tendon phenomenon is observed in cases where tin- 

 pyramidal tracts are degenerated, and indicates a heightened reflex excitability of the 

 lower spinal centres, perhaps reinforced by impulses from the cerebellum. The impor- 

 tance 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 cord, and especially the knee-jerk. How far this absence of tone is due to col- 



lateral 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 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 



Fio. 168. Diagram to show muscles and nerves rhythm from six to ten per second. 



The heightened tone of the muscle, 



under these conditions, and the ease 



,. , , . 



ih whlch anv . 8b g ht crease m 

 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.' 



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 ruptu're some of its fibres and per- 

 haps injure the adjacent joint surfaces. An ordinary reflex contraction 

 would be powerless to prevent this, since the mischief would be done before 

 the reaction could take place. But by means of the enormously rapid 

 mechanism of the * tendon reflex ' the muscles are able to react to any 

 sudden increase in their tension by an equally sudden contraction, which 

 saves the joint before the individual has even become aware of the strain. 



,,,,, . , 

 L3, lA, L5, third, fourth, and fifth lumbar roots; 



si, s2, first and second sacral roots. 



THE SPINAL MAN 



During the last few years opportunity has been given for the study in man of 

 the effects of complete isolation of the spinal cord. The results differ from those 

 observed in the monkey, much in the same way as these differ from those in the dog 

 or cat. 



