434 Mr. J. S. R. Russell. Circumstances under which [May 18, 



Hughlings- Jackson* has recorded an instance of exaggeration of 

 the knee jerk on one side after convulsions which affected chiefly the 

 leg on that side. This observer has also published a case in which 

 the knee jerks, absent in a case in which the posterior columns of the 

 cord were sclerosed, returned when the lateral pyramidal tracts 

 became sclerosed consequent on a cerebral haemorrhage. Dr. Jack- 

 son has also observed that the knee jerks are absent in conditions 

 attended by hyper- venosity of the blood in man. 



Beevorf found that after generalised convulsions in man the knee 

 jerks were absent or exaggerated according as the convulsions were 

 very severe or less so, and the former condition he ascribed to ex- 

 haustion of the lumbar centres consequent on their severe discharge 

 of energy. 



JendrassikJ has furnished us with a valuable method of eliciting 

 the knee jerk when it appears to* be absent in healthy persons. It 

 consists in making the person clench the hands or perform some 

 other violent movement at the moment when the patella tendon is 

 struck, when the resulting jerk is found to be increased. He con- 

 cluded that the knee jerk is a true reflex caused by mechanical 

 irritation of the nerves in the tendon ; that passive tension of the 

 muscle is necessary for its production ; that voluntary innervation of 

 the crural nerve lessens or prevents it ; and that innervation of the 

 sciatic favours it. 



Weir Mitchell, and Lewis,§ from a large number of investigations 

 in man, supplemented by a few in dogs and rabbits, concluded that 

 the knee jerk varies in health, may be exhausted by too much use, 

 and may increase from frequent excitation. All volitional acts may 

 increase it ; and weak innervation of the crural nerve increases it, as 

 does any form of innervation of the sciatic, while strong prohibits it. 

 Continual violent muscular actions, as of both arms and hands, 

 eventually enfeeble the knee jerk. Passive tension is not essential 

 for its production, moderate tension mechanically favours it, and 

 extreme tension destroys it even in spastic cases. An act of will 

 directed to a part functionally inert, or to amputated parts, reinforces 

 the knee jerk. Strong or weak stimulation of one sciatic in etherised 

 animals intensifies the knee jerk of the other leg ; and pressure on 

 the sciatic in man sufficient to cause pain and numbness diminishes 

 the knee jerk on that side. One knee jerk does not reinforce the 

 other. Touch has no effect on it, but all abrupt impressions such as 

 pain, heat, and cold anywhere on the skin increase it, as does a 



* ' Med. Times and Gaz.,' February, 1881 ; ' Brit. Med. Journ.,' July, 1891 ; 

 February and March, 1892. 

 f ' Brain,' Part XYII, 1883. 



X ' Deutsches Arch. f. klin. Med.,' vol. 33, p. 177, 1883. 



§ ' The Medical News of Philadelphia,' February 13th and 20th, 1886. 



