Activities of the Mid-Brain. 



147 



stimulated. These were on different circuits, and the two " indifferent electrodes " made 

 necessary were applied one to either foot. 



Stimulation of the peripheral nerves (ipsilateral and contralateral ulnars in the arm) 

 was performed in the usual bipolar method. 



For the proper examination of the movements of the arm in these experiments the 

 movements of an extensor (humeral part of triceps brachii) and of a flexor (supinator 

 longus) of the elbow were registered simultaneously. All the other muscles of the left 

 arm and shoulder were destroyed by motor paralysis. 



In the following descriptions the terms " ipsilateral " and " contralateral " are used in 

 reference to the left arm — the former therefore meaning (here) " a point on the left side 

 of the body," and the latter " a point on the right side." 



III. The State of the Monkey after Decerebration. 



After comparatively high decerebration (that is, when the neuraxis is 

 divided across slightly anterior to the anterior colliculi), or even when the 

 division is through the anterior colliculi, the animal is not perfectly 

 immobile. When the depth of narcosis has shallowed the eyelids are open, 

 and sometimes wide open. Winking frequently occurs, and the eyes 

 sometimes are moved. From time to time the animal slowly changes its 

 posture, the movements being like those of normal sleep. Owing to the fact 

 that the animal in these experiments has been carefully covered and propped 

 in a definite posture for the recording of the arm movements, it is not easy 

 to describe those postures accurately. But, if the attention is confined to 

 the movements of the left arm, it is seen that slow postural flexion and 

 extension occur from time to time. The flexor thus may slowly contract, 

 and, having reached its maximum of contraction, may there remain for 

 many minutes if undisturbed. And, similarly, the extensor may at other 

 times contract and remain contracted. The hind limbs may shew postural 

 extension (Sherrington's " decerebrate rigidity ") and, although the state of 

 the hind limbs has not systematically been examined, it has seemed that 

 they tend more frequently to demonstrate the extensor rigidity than do the 

 fore limbs. From time to time the head is uneasily moved, and the animal 

 seems to react (by closing the eyes) to loud and sharp sounds — although my 

 evidence on this point is not very clear. 



In short, the decerebrate monkey appears to be in a state which closely 

 resembles that of light sleep, and the fact that, in this condition, these slow 

 and maintained postural activities of flexion and extension in the arm may 

 occur after decerebration shews conclusively that they are conditioned by 

 centres below those in the cerebrum. 



