8oo THE SPINAL CORD. 



maintaining the normal tonus of the vasti-crureus extinguishes the knee-jerk. 

 The particular afferent channels on which the tonus and jerk depend in this 

 case, have been traced to arise in the muscle itself. 1 Cutaneous nerves seem 

 not to be the constituents of the afferent spinal root which in respect to mus- 

 cular tonus are of chief importance. 2 There is no asymmetry of facial expres- 

 sion, after section of the sensory root of the trigeminus, though in the rabbit 

 the pinna drops 3 on the side homonymous with the facial anesthesia ; but 

 there is facial asymmetry after section of one facial nerve. The myogram 

 obtained when the intact nerve of the quadriceps cruris is excited by a break 

 shock shows a relaxation line much slower and more prolonged than when 

 the cut nerve is similarly excited, and the relaxation line in the former case 

 never, as in the latter, dips below the base line with an inertia fling. 4 It 

 is the inertia fling attaching to the movements, that is so characteristic of an 

 ataxic limb. This has been admirably analysed by H. E. Hering 5 in the case 

 of his "Hebe-phenomen." in the frog. The contraction of the muscle when 

 its afferent path is intact seems to set into activity some mechanism, lost after 

 rupture of the arc, that damps the inertia swing of the muscular movement. 

 It may be that the tension developed in the muscle by its contraction excites 

 reflexly in it a condition of tonus. JS'ot every muscle seems, under all normal 

 conditions, to conform to the statement that there is a constant neural tonus 

 of reflex origin in it. When the elbow is well flexed, the biceps is so slack 

 as to feel toneless. When the frog's leg is fully flexed at knee and 

 extended at ankle, the exposed gastrocnemius can be seen to lie actually in 

 slack folds; on being cut it does not gape. It may be that the neural 

 tonus of skeletal muscles is only present in them so long as they are 

 stretched, and only in such as are to a certain degree under stretch. The 

 tonus will obviously be of chief advantage in such muscles as have to be 

 in readiness to contract; the reflexly maintained spinal "sitting" posture 

 of the frog entails, by passive stretch, tonus in the extensors, and holds 

 them ready for the next movement a spring. I^ot only is the posture of the 

 pendent limbs of the spinal frog altered from normal by section of the afferent 

 roots of the limb, but the sitting posture of the apsesthetic limb in the other- 

 wise normal frog is abnormal, in the sense that the limb is less flexed 

 at hip, and knee, and ankle. 6 The apsesthetic fore-limb is, on the other 

 hand, more flexed at elbow, and the digits are often bent under. The excit- 

 ability of the cord and centres, under the influence of convulsants, is much 

 depressed by section of the afferent roots ; 7 it seems that in the spinal frog 

 strychnin and picrotoxin 8 fail to produce convulsions if all the afferent 

 spinal roots be severed. Certainly, however, asphyxia can excite spinal con- 

 vulsions when all the afferent roots of the cord or portion of cord under 

 examination have been severed. 9 The peculiar condition of extensor spasm 

 of the limbs characterising decerebrate rigidity, after transection between 

 diencephalon and mesencephalon, does not ensue in a limb the afferent spinal 

 roots of which have been previously severed, and disappears at once in a limb 

 on the severance of the afferent roots of the limb. 10 The severance of other 

 afferent roots does not destroy the rigidity of the limb. On the other hand, 

 the section of an afferent root seems to make but slight change in the excit- 

 ability of the motor cells of the corresponding efferent root ; when tested by 



1 Sherrington, Proc. Roy. Soc. London, 1893, vol. lii. 



2 Mommsen, Virchoiv's Archiv, 1885, Bd. ci. S. 22. 



3 Filehne, Arch. f. Physiol., Leipzig, 1886, S. 440. 



4 Tschiriew, ibid., 1879, S. 82. 



5 Arch.f. d. ges. Physiol., Bonn, 1897, Bd. Ixviii. S. I. 



6 H. E. Hering, Arch.f. exper. Path. u. Pharmakol., Leipzig, 1896, Bd. xxxviii. S. 276. 



7 Luchsinger ; H. E. Hering, Arch.f. d. ges. Physiol., Bonn, 1893, Bd. li. S. 614. 



8 H. E. Hering, ibid., op. cit. 



9 Luchsinger and others ; also Sherrington, Phil. Trans., London, 1897. 



10 Sherrington, Journ. Physiol., Cambridge and London, 1898, vol. xxiii, p. 319. 



