ON THE 
INNERVATION OF ANTAGONISTIC MUSCLES* 
SIXTH NOTE. By C. S, SHERRINGTON, M.A., M.D., F.R.S. 
Machine-like regularity and fatality of reaction, although characteristic of spinal reflexes, 
is yet not exemplified by them to such extent that similar stimuli will always elicit from the spinal 
animal similar responses. This want of certainty as to response is an interesting difficulty 
attending the study of spinal reactions. The variation in the responses of the skeletal musculature 
manifests itself, not only in regard to the extent of the movement, but also in regard to the direction 
of the movement. 
Some of the factors determining the character of the reactions are factors contained within 
the stimulus. Important among these is the ' locm of the stimulus.' Thus it has long been known 
that the direction and other characters of the reflex movement are influenced by the mere location 
of the stimulus. Nevertheless stimuli identical in all respects, including locality, may evoke 
reflex movements of widely different, even of absolutely opposite, character. Such differences of 
response must be referred to differences obtaining at the time in the spinal organ itself. One 
cause for such differences seems indicated by the following observations : — 
The most usual, indeed the almost invariable, primary reflex movement of the hind-limb of 
the spinal dog (and cat), when spinal transection has been performed in the cervical or upper 
thoracic region, is flexion at hip, knee, and ankle ; the limb is 'drawn up.' This movement can 
be well obtained by, among other stimuli, the pressing of the pads of the digits upwards so as to 
extend the toe-joints, a stimulus that in some measure imitates the effect upon those joints of the 
bearing of the foot upon the ground under the animal's weight. Extension as a reflex result from 
this stimulus is, in my experience, never met in the homonymous limb in the early time after 
transection. When a certain period has elapsed, three weeks or more after transection, and shock 
has largely subsided, it becomes possible to, at times, obtain extension at hip as the primary 
movement in the homonymous limb. The pressing of the toe-pads upwards, spreading and 
extending the digits, elicits a sharp movement of extension at the hip, if at the time the initial 
posture of hip and knee be flexion. If the initial posture of hip and knee be extension, the primary 
reflex movement excited is, in my experience, invariably flexion. The reflex movement is, it is 
true, not unfrequently flexion even when the initial posture is one of flexion ; but it is, on the 
other hand, very frequently, and especially preponderantly in certain individual animals, extension. 
The passive assumption of a flexed posture at hip and knee seems to favour the reflex movement 
at those joints taking the form of extension. The influence of the posture of the ankle-joint upon 
* Reprinted from the ' Proceedings of the Royal Society of London,' 1899. 
