CHAP, vi.] THE BRAIN. 621 



a centre. And this form again may easily pass into a simply rolling 

 movement. In yet another form the animal rotates over the trans- 

 verse axis of its body, tumbles head over heels in a series of somer- 

 saults; or it may run incessantly in a straight line backwards or 

 forwards until it is stopped by some obstacle. These latter forms 

 of forced movements are frequently seen after injury to the corpora 

 striata even when a very limited portion of their grey matter is 

 affected. Lastly, many, if not all, these various forced movements 

 may result from injuries which appear to be limited to the cerebral 

 cortex. 



Attempts have been made to explain the rotatory movements 

 by reference to unilateral paralysis or to spasm of various muscles 

 of the body caused by the cerebral injury ; and in the case of the 

 'circus' movements with partial hemiplegia, which follow upon 

 injury to the corpora striata or other parts, the explanation that 

 the animal in progressing forward naturally bears on its paralysed 

 or weak side seems a valid one; but the movements may frequently 

 be witnessed in the complete absence of either paralysis or spasm, 

 and cannot therefore be always so explained. On the other hand, 

 if the views urged just now concerning the nature of the coordina- 

 ting mechanisms of the brain are true, it is evident that they afford 

 a general explanation of the phenomena, though our present know- 

 ledge will not permit us to explain the genesis of each particular 

 kind of movement. Such gross injuries as are involved in dividing 

 cerebral structures or in injecting corrosive substances into the 

 midst of cerebral organs, must of necessity, either by irritation or 

 otherwise, seriously affect the transmission not only of afferent 

 impulses in their cerebral course, but also of central impulses, in- 

 hibitory and the like, passing from one part of the brain to another; 

 and must therefore seriously affect the due working of the general 

 coordinating mechanisms. The fact that an animal can, at any 

 moment, by an effort of its own will, rotate on its axis or run 

 straight forwards, shews that the nervous mechanism for the execu- 

 tion of those movements is ready at hand in the brain, waiting only 

 to be discharged ; and it is easy to conceive how such a discharge 

 might be affected either by the substitution of some potent intrinsic 

 afferent impulse for the will or by some misdirection of the volitional 

 impulses. Persons who have experienced similar forced movements 

 as the result of disease report that they are frequently accompanied, 

 and seem to be caused, by disturbed visual or other sensations; 

 thus when they suddenly fall forward they say that they do 

 so because the ground in front of them appears to sink away 

 beneath their feet. Without trusting too closely to the interpreta- 

 tions the subjects of these disorders give of their own feelings, we 

 may at least conclude that the disorderly movements are in many 

 cases due, not to any paralytic or other failing of the simple muscu- 

 lar instruments of the nervous system, but to a disorder of the 

 coordinating mechanism, which in many cases is itself the result of 



