THE KNEE-JERK AND ALLIED PHENOMENA. 871 



posture of slight flexion at hip and knee, and all its muscles lie at 

 rest, a tap on the patellar ligament evokes a short contraction of the 

 quadriceps cruris, 1 so that extension at the knee ensues. This is the 

 phenomenon known as the "knee-jerk." Others similar exist in other 

 pieces of musculature, e.g. at the tendo Achillis for the triceps surae, 

 and at the extensor tendons of the wrist, and the masseter. The knee- 

 jerk may serve as their type. Muskens 2 finds deficiency of jerk 

 a usual, though not invariable, accompaniment of subnormal tension 

 of the Achilles tendon ; conversely he finds hypertonus of the triceps 

 surce to co-exist with exaggerated jerk. He estimates degree of tonus 

 by measurement of the pressure required for displacement of the 

 tendon, the ankle resting in semi-flexion. The study of the time-phases 

 of the muscular contraction has shown it to have all the characters of 

 a simple twitch (Brissaud, 3 Eulenberg, 4 MacWilliam 5 ). The interval 

 elapsing between the application of the tap and the appearance of the 

 contraction is too short to permit of the reaction being considered a 

 reflex 6 vid the afferent root, the cord, and the motor root cells. The 

 time is about the same as that of the contraction response on direct 

 electrical excitation of the muscle, i.e. less than a quarter of that of the 

 briefest reflex time. In accord with Sternberg's supposition that the 

 mechanical excitability of the muscle varies directly as the jerk, Muskens 7 

 observes the tap elicit in the hypotonic calf muscle a slow and merely local 

 contraction instead of the normal quick response of the whole muscle. 



A sine qud non for the elicitation of the "jerk" is slight tension of 

 the muscle which is to respond ; and the stimulus to the reaction is a 

 sudden slight passive increase of the tension. Gowers has suggested, 8 

 therefore, that the "jerks" be termed "myotatic" contractions (rdnxog, 

 extended). They are sometimes termed " tendon-phenomena," but 

 tendons are not essential. The terms " tendon reflexes " and " deep 

 reflexes " have obvious objections as applied to the "jerk phenomena," 

 for they are not proved to be reflex, although there do exist, in addition, 

 undoubted reflexes from tendons and other deep structures. 



On analysing 9 the spatial distribution of the phenomenon in the 

 muscles of the thigh, it is found to involve chiefly the vastus medialis 

 and adjacent crureus, and when not strong to be entirely limited to 

 them. When isolated in those muscles, it is at once abolished by 

 severance of the afferent nerve fibres belonging to those particular 

 muscles, whereas combined severance of all the other afferent nerve paths 

 of the limb does not abolish it. The phenomenon, though not a reflex 

 act, is therefore dependent on a reflex spinal arc, 10 the afferent part of 

 which is based on sense organs in the muscle itself and its appurtenances. 



1 C. Westphal, Arch. f. Psychiat., Berlin, 1875, Bd. v. S. 803 ; W. Erb, Berl. klin. 

 Wchnschr., 1875, S. 26. 2 Neural. CentralbL, Berlin, 1899. 



3 " Recherches physiol.," Paris, 1880. 4 Ztschr.f. klin. Med., Berlin, Bd. iv. S. 179. 



5 CentralbL f. d. med. Wissensch., Berlin, 1887. 



6 Waller, Lancet, London, 1881, vol. ii. p. 83 ; Brain, London, 1881, vol. iii. p. 179 ; 

 Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 384 ; de Watteville, Brit. Med. 

 Journ., London, 1882, vol. i. p. 736 ; Brain, London, 1886, vol. viii. p. 516 ; Gowers, Med.- 

 Chir. Trans., London, vol. Ixii. p. 269 ; Burckhardt, Festschr. z. Andenken an A. v. Hatter 

 dargebracht, Bern, 1877 ; Eulenberg, loc. cit. ; Jendrassik, Neurol. CenlralbL, Leipzig, 1885, 

 S. 412 ; Gotch, Journ. Physiol., Cambridge and London, 1897, vol. xix. ; also, Rosenheim, 

 van T. Meulen, etc. 



7 J. L. L. Muskens, loc. cit. 8 "Diseases of the Nervous System," London, 1886, vol. i. 

 9 Sherrington, Journ. Physiol., Cambridge and London, 1892, vol. xiii. ; Proc. Roy. 



Soc. London, 1892, vol. li. 



10 Prevost, Rev. med. de la Suisse row., Geneve, 1881 ; Westphal, Berl. klin. Wchnschr., 



