1093 
mity is moved passively, in which case one set of muscles is 
stretched, another group being shortened. 
A living animal’s muscles at rest are generally not entirely relaxed. 
A slight degree of elastic tension, called tonus, persists. Tonus is 
for the greater part caused and restrained by regulating impulses, 
originating from peripheral sensory and higher motor neurones. 
Also the cortex and the gangliongroups of the cerebrum, the laby- 
rinth, the cerebellum control and influence the muscular tonus. 
Tonus varies under different circumstances, but it adapts itself auto- 
matically to the rate of stretching of the muscle. If the muscle be 
slowly stretched by a passive movement of the limb, its form changes. 
The muscle grows longer and thinner. But its tension does not change 
at the same rate. Only if the stretching be carried very far or happens 
within a very short space of time its elastic tension grows appreciably. 
With a passive shortening of the muscle something analogous 
occurs. The length diminishes, the diameter increases but the tension 
adapts itself automatically to the new condition, and the muscle 
does not become slackened so far as to show folds or furrows. 
This adaptibility only persists as long as the muscle remains in 
contact with the intact nervous system. As soon as the muscle is 
freed from its nerve, its reflex-tonus disappears and it seems to 
behave simply as an elastic string, in which a definite tension 
corresponds to a definite length. The action of the nervous system 
seems to equalise the tension for different lengths and causes the 
resting-length of an innervated muscle to be a varying quantity. 
If the passive shortening of a muscle is effected within a very 
short time we sometimes observe a genuine contraction of the muscle 
followed by the thickening caused by the reflextonus. This pheno- 
menon I have called the shortening reflex. 
If the foot be passively and somewhat forcibly extended (= dorsal 
flexion) we are sometimes able to see and feel a very short con- 
traction of the m. tibialis antieus. After this contraction the tonus- 
thickening becomes visible. The contraction cannot be elicited in 
every healthy individual, and even where it is to be found, it is 
often rather difficult to obtain. We get it most easily in the tibialis 
anticus by extending the foot. In some cases I have also found it 
in other muscles, as in the flexors of the arm, the flexors of the 
leg after flexing the arm or the leg. 
I have recorded the phenomenon with a special apparatus, con- 
structed some 9 years ago for recording the foot-clonus. The difficulty 
was to rigidly attach a pair of Maregy’s tambours to the bony parts 
of the leg, so as not to become displaced by the violent movements 
