THE KNEE-JERK AND ALLIED PHENOMENA. 873 



Westphal 1 noted that in patients in whom, after epileptic seizure, the knee-jerk 

 was inelicitable, it became elicitable after a reflex had been evoked from the leg by 

 pricking the plantar skin. The duration of the reinforcement of the jerk by a local 

 stimulus is variable, but may be long persistent if the stimulus have been strong. 



Faradisation of the central end of the contralateral sciatic nerve reinforces 

 the knee-jerk. 2 Excitation of the central end of the nerves of the flexor 

 muscles of the knee 3 (which in the cat are collected into a trunk free 

 from cutaneous nerve fibres) strongly inhibits the jerk, causing at the same 

 time a relaxation of the extensor muscles of the knee if they are then— e.g. 

 under decerebrate rigidity — in tonic contraction. Mechanically stretching the 

 knee flexors — isolated in such a way as to prevent the pull upon them affecting 

 the freedom of the knee-joint — similarly inhibits the jerk. On discontinuance 

 of the stimulus, the jerk is much reinforced, and soon is so even during the 

 stimulus, if that be prolonged beyond a certain time. Section of the nerves of 

 the knee flexors is followed by a long lasting, slight augmentation of the jerk, 4 

 so also is section of the whole sciatic nerve. Faradisation of the cutaneous 

 nerves 5 of the limb also generally depresses the jerk, but not so regularly or 

 potently as the excitation of the nerves from the muscles antagonistic to the 

 knee extensors. Bowditeh and Warren 6 found sensible stimuli to skin and 

 mucosae, e.g. sudden application of cooled air to conjunctiva, skin of neck, and 

 nasal passages, reinforce the jerk if applied simultaneously with or not more 

 than 0*2 second to - 4 second before the tap on the patellar tendon. If applied 

 earlier tban that, they exerted an inhibitory or depressor influence, which 

 reached its maximum about 1 second after application of the sensible stimulus. 

 Stimulation of the skin of the knee, either of the side whence the jerk was 

 elicited or of the contralateral side, caused little reinforcement, but when applied 

 at appropriate intervals, a distinct inhibition was noticeable even when the 

 stimulus preceded the tap for the jerk by fully three seconds. With other 

 stimuli tried, no effect on the jerk was traceable, if the sensible stimulus pre- 

 ceded the knee tap by two seconds. A sudden sound reinforced the jerk, and 

 never depressed it ; it reinforced most when it preceded the tap by 0'2 second to 

 0'3 second. A flash of light also reinforced ; the best interval being 0*1 second 

 to 0"3 second; even at appropriate intervals it caused little inhibition. 



Arrest of the circulation in the lumbar region of the cord was shown by 

 Prevost 7 to first exalt, and later depress and abolish the knee-jerk. Stimulation 

 of the peripheral end of the cardiac vagus in the dog and monkey similarly 

 exalts the jerk, 8 no doubt by impeding the circulation. Chloroform and ether 

 inhalation depress the jerk, and finally extinguish it, the speed of extinction 

 being about the same in the spinal as in the intact animal, but in the latter, after 

 cessation of the administration of the anaesthetic, recovering the more quickly. 9 



In the monkey, spinal transection usually depresses the jerk for a longer time 

 than in the cat and dog. 10 Occasionally in the monkey, after transection at the 

 lower thoracic region, the jerk is not elicitable for a week or so after transection. 

 In the cat and dog it is elicitable in a quarter of an hour or less. After transection 

 in the thoracic region, the jerk does not disappear immediately, but only in the 

 course of a minute or so ; u similarly, the knee-jerk has been elicited immediately 

 after decapitation in man. 12 In man, complete transverse destruction of the cord, 

 even well above the lumbar region, seems usually to permanently abolish the knee- 

 jerks; but a certain number of well-authenticated cases show that it is not always so. 



1 Arch.f. Psychiat., Berlin, 1875, Bd. v. S. 803. 2 Sternberg, op. tit. 



3 Sherrington, Proc. Roy. Soc. London, 1892. 4 Sherrington, ibid. 



5 Sternberg, op. cit. fi Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 25. 



7 Op. cit., 1881 ; see also Hughlings Jackson and R. Russell, Brit. Med. Journ,.. London, 

 1893, vol. ii. 



8 Sherrington, Journ. Physiol., Cambridge and London, 1892, vol. xiii. 



9 Sherrington, ibid. 10 Sherrington, ibid. 



11 Sherrington, ibid. 12 Barb4, Compt. rend. Soc. de biol., Paris, 1885, p. 533. 



