560 Prof. C. 8. Sherrington. On Reciprocal [Aug. 10, 
stretching the muscle; regulation in the other direction is shown by the 
“shortening reaction” which is a reflex initiated by approximating either 
passively or actively the ends of the muscle. By these means there is 
conferred upon the tonus of the extensor muscle a quality of plasticity. This 
plasticity enables the tonus to serve as an adjuvant to the reflex contraction 
' of the muscle and also to assist the control of the muscle by its antagonist 
in the play of opposed movements succeeding each other at the joint upon 
which both muscles act. 
III. The “lengthening reaction” and “shortening reaction” are readily 
studied in the vasto-crureus preparation above mentioned, but they are also 
well observable in other muscles exhibiting the rigidity of the decerebrate 
condition, If the preparation of the hind limb be modified so as to yield a 
gastrocnemius or soleus preparation, both reactions are evident in those 
extensors of the ankle. The preparation affords opportunity for determining 
a point less easily resolved by the vasto-crureus preparation. The nerve of 
vasto-crureus is credited with supplying a twig to the knee-joint and the 
question arises whether afferent nerve-fibres from the joint are not responsible 
for initiating the reactions described. To answer this, the isolation of the 
vasto-crureus from the knee-joint is required. When this is done, I find the 
reactions still obtainable. The afferents from the joint are therefore not. 
essential for the reactions; they may, however, be contributory to them. In 
the case of the nerves to gastrocnemius and soleus, these do not supply twigs 
to either knee or ankle joint. The obtaining of the lengthening and 
shortening reactions in the gastrocnemius-soleus preparation shows, therefore, 
that in this case articular afferent nerves are not even contributory to them. 
The “ lengthening” and “shortening ” reactions are also observable in the 
extensor muscles of both the shoulder and elbow; probably they are so also. 
in the extensors of the neck and tail and the retractors of the head. 
IV. These reactions observable in decerebrate rigidity are also obtainable 
after spinal transection in the dog. The extensor muscle of the knee exhibits. 
considerable tonus in the dog after spinal transection in the thoracic region. 
when time is allowed for subsidence of the spinal shock immediately 
consequent on the operation. In the knee-extensor of the spinal dog the 
“lengthening reaction ” is easily obtained, and it is then more regularly than in 
the decerebrate cat accompanied by crossed extension at the opposite knee. 
The crossed extension of the opposite knee induced by passive flexion of the 
other knee has been observed by Philippson* in his spinal dogs and has been 
* ‘Travaux du Laboratoire de Physiologie, Institut Solvay, Bruxelles,’ published by 
P. Heger, vol. 7, part 2, p. 31, 1905. 
