148 



Mr. Graham Brown. 



IY. Electrical Stimulation of Regions in the Cross-section of the Mid-brain 

 Dorsal to the Area of the Corticospinal Tract. 



Unipolar stimulation of the cross-section of the mid-brain at the level of 

 the anterior colliculi — when applied at a point in an area which lies dorsal 

 to that of the cortico-spinal tract — gives a definite movement of the arms. 

 The focal point in this area — that is, the most excitable point in it — lies 

 ventral to the central canal. The area includes that of the nucleus ruber 

 and of the posterior longitudinal bundle. 



Stimulation within this area upon one side of the mid-brain is accom- 

 panied by the assumption of a definite posture on the part of the animal. 

 The back of the head is twisted towards the same side and the face away 

 from it, the neck is bent concave to the same side (sometimes the face seems 

 to be turned to the same side). The arm of the same side is flexed, that of 

 the opposite side is extended. The lower limb of the same side is extended 

 and the opposite one flexed (but at one period in an experiment in which this 

 was usually the case I observed the ipsilateral hind limb to be flexed and 

 the contralateral to be extended). The tail is bent to the same side. I have 

 not been able carefully to examine the movements of the trunk. 



"When stimulation has ceased the posture is maintained. Thus if the 

 attention be directed to the movements of the arm muscles alone it is found 

 that the ipsilateral flexion (or contralateral extension) may outlive the 

 evoking stimulus for several minutes. 



1. The Ipsilateral Reaction. — When the movements of individual muscles 

 in the arm are examined {e.g. fig. 2) it is found that stimulation of the 

 ipsilateral area is immediately followed by a sharp flexor contraction. This 

 soon attains a maximum at which it persists throughout the application of 

 the stimulus. If extensor tonus is in beino- the flexor contraction is accom- 

 panied by reciprocal extensor relaxation. Sometimes during a long applica- 

 tion of the exciting stimulus an extensor contraction — accompanied by slow 

 flexor relaxation — may appear late in the period of stimulation. On with- 

 drawal of the stimulus there is usually no relaxation of the state of flexor 

 contraction, which then may persist for many minutes. This is the typical 

 reaction, and by far the most common. But flexor relaxation occasionally 

 occurs at the termination -of stimulation, and this may be followed by an 

 extensor terminal contraction which is comparatively well maintained. All 

 these types of reaction have been seen 10 months after the division of all 

 the posterior spinal roots supplying the left arm. 



2. The Contralateral Reaction. — The result of stimulation of the contra- 

 lateral area is to evoke a contraction in the extensor muscle {e.g. fig. 4). 



