874 THE SPINAL CORD. 



The spinal reflex mechanism connected with the knee-jerk of each side is 

 unilateral, and lies in its own half of the cord. In the cat and dog it lies in 

 the fifth and sixth lumbar segments ; in the monkey (Macacus), in the fourth 

 and fifth, chiefly the latter; and in man in the third and fourth, chiefly 

 the latter. It has been experimentally shown that the lumbar cord can be 

 split lengthwise along the median sagittal plane without impairment of the 

 jerk of either side. 1 Spinal transection above the lumbar region renders the 

 jerk, after the short period of depression, more brisk than normal. After 

 transection at the junction of diencephalon and mesencephalon, the briskness of 

 the knee-jerks is enormously increased. A single tap on the tendo patellce will 

 excite a series of jerks, and often the relaxation of the muscle between the jerks 

 is only imperfect. 2 Such a condition of jerk is sometimes called " tremospasm." 

 Raising the intracranial pressure has been noted to have a like effect on the "jerk." 3 

 Cortical excitation has also been found to exalt the jerk. After ablation of the 

 Rolandic cortex, the contralateral knee-jerk becomes usually the more brisk. 4 



Jendrassik 5 noted that a willed movement of the arm, if carried out by 

 the subject at the time of the elicitation of the knee-jerk, augments it. It 

 was later remarked 6 that to get the full effect of the reinforcement, the patellar 

 tap must be delivered at just the right moment after the reinforcing act. 

 Bowditch and Warren, 7 in their research, found the reinforcing influence to last 

 only from 0*22 second to 0*6 second, and to be followed, in most persons, by a 

 more persistent period of inhibitory influence, the normal degree of jerk being re- 

 covered in 1 '7 seconds to 2 *5 seconds. The willed movement of another part, no 

 doubt, removes the attention from the limb whence the jerk is to be obtained ; 

 in directing attention to any part of the body, there is a strong, in some persons 

 an irrepressible, tendency to innervate the skeletal musculature of the part. 

 How it is that even feeble excitation (via cerebral paths) of the motor root cells 

 prevents the muscle from giving the twitch, in response to the patellar tap, is 

 not clear. The fluctuation in the condition of the motor root cell, from minute 

 to minute, as wakeful life proceeds, is well illustrated by the fluctuation- of the 

 knee-jerk, as for instance observed by Lombard 8 under ordinary daily events. 

 Swallowing, talking, the hearing of a child crying in a near room, the 

 declamation of a poem, an itching of the ear, events such as these seem to be 

 mirrored as regards their influence on the motor root cells for the skeletal 

 musculature by reinforcements of the knee-jerk. In sleep the knee-jerk is, when 

 sleep is deep, usually unobtainable, 9 but sensory stimuli, too feeble to wake the 

 sleeper, make the jerk elicitable for a short interval. The perturbations of the 

 condition of the extensor muscle of the knee and its local reflex arc brought 

 about by sensory stimuli, customarily regarded as without consequence to, or 

 almost indifferent as regards that region, show to what extent the whole nervous 

 system is consolidated, and how continually its state of tension is braced or 

 slackened by stimuli whose influence we do not recognise, or recognise only in 

 one partial aspect. Fatigue, hunger, and enervating weather decrease the 

 average knee-jerk, while rest, nourishment, and invigorating weather increase 

 the average knee-jerk. In the winter frog the knee-jerk is difficult to elicit, 



1 Sherrington, Journ. PhysioL, Cambridge and London, 1892, vol. xiii. 



2 Brown-Sequard, Journ. de la physiol. de I'homme, Paris, 1858, tome i. p. 472 ; 

 Ordenstein, These, Paris, 1867 ; Bouchard, Arch. g<*n. de mid., Paris, 1866, tome ii. p. 290; 

 Dubois, These, Paris, 1868; Charcot et Joffroy, Arch, de physiol. norm, et path., Paris, 

 1869, tome ii. ; Strauss, "Des Contractures," These, Paris, 1875 ; Erb, Arch. f. Psychiat., 

 Berlin, 1875, Bd. v. S. 792; C. Westphal, ibid., Bd. v. S. 803. 



3 Adamkiewicz, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1883 (3), Bd. xxxviii. S. 231. 



4 Ziehen, Arch. f. Psychiat., Berlin, 1890, Bd. xxi. S. 863. 



5 Neural. Centralbl., Leipzig, 1885, S. 412. 



6 Mitchell and Lewis, Med. News, Phila., 1886, Feb. 13 ; Lombard, Journ. Physiol., 

 Cambridge and London, 1889, vol. x. p. 122. 7 Loc. cit. 



8 Journ. Physiol., Cambridge and London, loc. cit.; Am. Journ. Psychol., 1888, 

 vol. viii. ; Arch. f. PhysioL, Leipzig, 1889, Suppl., S. 292. 



9 Bowditch and Warren, loc. cit. 



