872 



THE SPINAL CORD. 



A great interest attaches to the knee-jerk and similar reactions as indices 

 of the condition of the nervous system. Extinguished when the neural tonus 

 of the muscle is extinguished (e.g. by complete rupture of the reflex arc) or is 

 replaced by the active innervation of intentional movement, these "jerk" 

 phenomena are indicators delicate enough to show many shades of spinal state. 

 Under apparently like circumstances, there are, in any large collection of 



healthy individuals, not in- 

 considerable differences of 

 knee-jerk. In a very few 

 persons, apparently healthy, 

 the knee-jerk is at times 

 inelici table x even under most 

 favourable conditions. Of 

 alterations in the jerk, some 

 are referable to conditions 

 directly affecting the local 

 reflex arc itself. Thus, a 

 second tap given soon after 

 a first will generally evoke a 

 more ample jerk. 2 A series 

 of taps too feeble to singly 

 evoke the jerk, will, when 

 repeated at intervals from a 

 fifth to a tenth of a second, 

 become efficient. Conversely, 

 after the jerk has been 

 elicited for a number of 

 times, it tends to fail for a 

 short period, despite the 

 taps being continued as be- 

 fore. Local fatigue of the 

 extensor muscles diminishes 

 the jerk ; 3 general fatigue of 

 the nervous system 4 at first 

 increases, but if severe, later 



Fig. 359. — Scheme of the arrangement of the nerves and 

 muscles of chief importance in the knee-jerk, as 

 determined in the Macaque monkey. The numbers 

 4 1,-8 l indicate the post-thoracic spinal roots of 

 Macacus, and the numbers in brackets the corre- 

 sponding lumbar and sacral roots of man. The 

 arrows pointing to the right indicate efferent paths, 

 those pointing to the left afferent paths. 



diminishes the jerk. The 

 rendering of the limb bloodless by the Esmarch bandage causes the knee-jerk 

 in about fifteen minutes to disappear. 5 " Pathic " stimuli applied to the 

 skin increase the jerk, and friction of the skin of the leg," and the cold 

 bath 8 , are means of increasing the jerk in patients in whom it is defective. 



1881 ; Schreiber, Arch. f. cxper. Path. u. Pharmakol. , Leipzig, 1885, Bd. xviii. S. 270 ; 

 Sherrington, Proc. Roy. Soc. London, 1892, vol. li. ; Schultze and Fiirbringer, CentralbL 

 f. d. med. Wissensch., Berlin, 1875, S. 929. 



1 Eulenberg without the now known reinforcements failed to elicit the jerk in sixteen 

 out of 238 healthy children ; Pelizseus and Remak with use of reinforcements found the 

 jerk present in each of 2403 healthy children examined in succession, Arch. f. Psychiat., 

 Berlin, 1885, Bd. xiv. S. 402 and 167. 



2 Heller, Bcrl. liin. Wchnschr., 1886, S. 903; Meyer, ibid., 1888, S. 23; Schreiber, 

 loc. cit. 



3 Muhr, CentralbL f. Nervenh. u. Psychiat. , Coblenz u. Leipzig, 1878 ; Jendrassik, Devtschcs 

 Arch. f. Min. Med., Leipzig, Bd. xxxiii. S. 177 ; Eisenlohr, Festschr. /. Erbffn. d. neu 

 allgcm. Krankcnh. z. Hamb. Eppendorf., 1889; Sternberg, "Sehnenrerlexen," Wien, 1893. 



4 Brissaud, op. cit. ; Marmian, Riv. din. di Bologna, 1884, p. 415; Schuster, " Diagnostik 

 d. Rtickenmarkskrankheiten," Berlin, 1884; Erlenmeyer, " Ueber statische Reflexkrampfe, " 

 Leipzig, 1885 ; Sternberg, CentralbL f. Physiol., Leipzig u. Wien, 1887, Bd. i. S. 81. 



8 Sternberg, "Die Sehnenreflexen," Wien, 1893, S. 69. 



6 Weir Mitchell and Lewis, Med. News, Phila., 1886; Jarisch u. Schiff, Med. Jahrb., 

 Wien, 1882. 



7 Schreiber, op. cit. 8 Dunge, "Diss.," Bonn, 1889. 



