162 



Activities of the Mid-Brain. 



some phenomena of interest are presented. Of these the first is the 

 characteristic tendency of the reactions to be followed by after-discharge 

 (contralateral, extension ; ipsilateral, flexion). The reflexes evoked are 

 essentially postural. This attribute disappears neither after mesial longi- 

 tudinal division of the mid-brain nor immediately after complete removal of 

 the cerebellum. Postural tonus of a perfect maintenance may be evoked in 

 the absence of the cerebellum by appropriate stimulation of these tracts. It 

 is possible that their activity is one of the chief factors in the great postural 

 reflexes, and that the cerebellum plays upon them but is not itself the 

 originator of the postural after-discharges. That the after-discharges do 

 slowly disappear after removal of the cerebellum may be due either to shock 

 or perhaps to the removal of a function of the cerebellum in maintaining the 

 proper activity of these paths and centres. 



The second point of interest is the mutual antagonism of the right and left 

 dorsal focal points. This, seen either in immediate or successive compound- 

 ing of the two, does not disappear either after mesial longitudinal division of 

 the mid-brain or after complete removal of the cerebellum. The point of 

 common antagonism is below the mid-brain, and it may be surmised that it 

 lies at as low a level as that of the spinal centres. 



A third point of interest is that the reactions — with their typical after- 

 discharges — may occur many months after division of the dorsal spinal roots 

 of the arm. That is to say, appropriate stimulation in the region of the mid- 

 brain may evoke an extensor postural tonus or a flexor postural tonus. 

 Sherrington has found that the " decerebrate rigidity " which occurs after 

 removal of the cerebrum does not occur in a " de-afferented " limb, but the 

 fact that a condition which at any rate very closely resembles this state may 

 be evoked in such limbs seems to point to the conclusion that the absence of 

 this postural tonus in the decerebrate " de-afferented " animal is due to the 

 failure of the ascending impulses from the limb which normally play — 

 however indirectly — upon these mechanisms of the mid-brain, and that the 

 mechanisms themselves if properly activated are still able to induce the 

 tonus. 



One point more may be referred to — the activity of the cortico-spinal tract. 

 The activity of the posterior longitudinal bundle (?) seems essentially to be 

 postural. That of the cortico-spinal tract seems essentially to be non-postural. 

 When the stimulus is stopped the reaction at once fails, and that with great 

 suddenness. When the flexion reaction of the contralateral crus cerebri is 

 pitted against the flexor after-discharge of the ipsilateral dorsal focal point 

 (posterior longitudinal bundle ?) the phenomena are of great interest. There 

 occurs during stimulation of the crus immediate augmentation of flexion, and 



