1020 THE MUSCULAR SENSE. 



Another influence exerted by " afference " (" Centripetalitdt," H. E. 

 Hering) upon the motor innervation of skeletal muscle has to be remem- 

 bered. The blocking all afferent channels lowers the muscular tonus, 

 as seen in the excessive passive mobility of the limb, and in the delay of 

 rigor mortis in its muscles. 1 



The mechanical tension of a muscle which results from its reflex 

 tonus seems important 2 for the proper execution of its role in a combined 

 movement. The action of a muscle A, if unprevented by external 

 resistances, inevitably produces centripetal impulses, which react on the 

 neural tonus, and therefore on the mechanical tension of its antagonist 

 muscles B. More than that, the action of the muscle A, if unprevented 

 by external resistance, inevitably not only stretches its antagonist B 

 passively, but in so doing causes a discharge of centripetal impulses in 

 B, by mechanically exciting B's end-organs, and that discharge modifies, 

 probably usually augments, the neural tonus of B. This regulates the 

 course of the movement, and is preparatory to a next step in the sequence 

 of movement, usually of the reverse direction to the initiatory. 



Probably, however, there are other factors still in the ataxy of an 

 apassthetic limb. Its ataxy is different to that of cerebellar disturbance, 

 and that of disseminated sclerosis. Analysis of it discovers disturbance 

 in the extent, the direction, and the speed of the movement as 

 compared with a normal movement of similar intention. The apses- 

 thetic limb of the frog is flung upward, much as the leg of the tabetic 

 patient is in walking advanced too high and too far. 3 I have seen the 

 apiesthetic fore-limb of a cat, when raised to assist the mouth in the 

 management of a large morsel, oftentimes miss the cat's mouth, and even 

 its head altogether. 4 Similarly in the working of the hand, although it 

 itself retain a large part of its sensorial innervation, if the rest of the 

 limb be apsethestic, it often misses the object at which it grasps, and 

 may pass an inch or so to one side of its intended goal. 5 The limb can 

 be seen to swerve from side to side in the progress of its movement. 

 The speed of normal willed movements has been shown to be uniform 

 from soon after start to close upon finish. 6 The jerky movements of 

 afferent ataxy are obviously not carried out with even approximately 

 uniform velocity. 



The centripetal impulses, loss of which produces loss of skeletal 

 tonus, arise not in the skin but in the deep tissues. Those afferent 

 channels, rupture of which produces this ataxia, are not princi- 

 pally cutaneous. A frog in which the hind-limbs have been rendered 

 apsesthetic, exhibits marked ataxia in those limbs ; but a frog from 

 which the skin of the hind-limbs has been removed, 7 moves with no 

 obvious want of precision. The bird in which the skin of the foot has 

 been insensitised by nerve section, will sleep perfectly balanced on the 

 insensitive foot. 8 The horse which goes lame from tenderness of the 

 hoof, trots normally after neurotomy has rendered the whole foot insen- 



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



2 B. Stilling, Arch. f. physiol. Heilk., Stuttgart, 1842, Bd. i. S. 97, and many later ob- 

 servers, especially H. E. Hering, C. Bell, 0. Bastian, Cyon, S. Exner, Mott and Sherrington. 



3 H. E. Hering, Arch./, exper. Path. u. Pharmalol, Leipzig, 1896, Bd. xxxviii. S. 276 ; 

 and Arch./, d. ges Physiol, Bonn, 1898, Bd. lxx. S. 559. 



4 Sherrington, Journ. Physiol., Cambridge and London, vol. xxii. 1898, p. 319. 



5 H. E. Hering, Neurol. Centralbl, Leipzig, 1897, Bd. xvi. 



6 Loeb and Koranyi, Arch. f. d. ges. Physiol, Bonn, 1890, Bd. xlvi. S. 101. 



7 A. Bernard, " Lecons sur la physiol. du systeme nerveux," Paris, 1858, p. 254. 



8 Chauveau, Brain, London, 1891, vol. xiv. 



