650 THE BRAIN. 



forced movements may result from injuries which appear to be confined to 

 the cerebral cortex. 



When the phenomena are well developed, every effort of the animal brings 

 on a movement of this forced character. Left to itself and at rest the ani- 

 mal may present nothing abnormal, its posture and attitude may be quite 

 natural ; but when it is excited to move or when it attempts of itself to move, 

 it executes not a natural movement, but a forced one, turning round or roll- 

 ing over as the case may be. In severe cases the movement is continued 

 until the animal is exhausted ; when the exhaustion passes off the animal 

 may remain for some little time quiet, but some stimulus, intrinsic or ex- 

 trinsic, soon inaugurates a fresh outbreak, to be again followed by exhaustion. 



In some of the milder forms, that for instance of the circus movement 

 with a long radius, the curved character of the progression appears simply 

 due to the fact that in the effort of locomotion volitional impulses do not 

 gain such ready access to one side of the body as to the other, the injury 

 having caused some obstacle or other. Hence the contractions of the mus- 

 cles of one side (the left, for instance) of the body are more powerful than 

 the other, and in consequence the body is continually thrust toward the other 

 (the right) side. As is well known, we ourselves, when our walk is not 

 guided by visual sensations, tend to describe a circle of somewhat wide 

 radius, the deviation being due to a want of bilateral symmetry in our limbs ; 

 and the above circus movement is only an exaggeration of this. 



But the other more intense forms of forced movements are more compli- 

 cated in their nature. No mere blocking of volitional impulses will explain 

 why an animal whenever it attempts to move rolls rapidly over or rushes 

 irresistibly forward or backward. It is not possible with our present know- 

 ledge to explain how each particular kind of movement is brought about ; 

 and, indeed, the several kinds are probably brought about in different ways, 

 for they differ so greatly from each other that we only class them together 

 because it is difficult to know where to draw the line between them. But 

 we may regard the more intense forms as illustrating the complex nature of 

 what we have called the coordinating machinery, the capabilities of which 

 are, so to speak, disclosed by its being damaged. Such gross injuries as are 

 involved in dividing cerebral structures or in injecting corrosive substances 

 into this or that part of the brain, must, of necessity, partly by blocking the 

 way to the impulses which in a normal state of things are continually pass- 

 ing from one part of the brain to another, partly by generating new unusual 

 impulses, seriously affect the due working of the general coordinating machin- 

 ery. The fact that an animal can, at any moment, by an effort of its own 

 will, rotate on an axis or run straight forward, shows that the nervous 

 mechanism for the execution 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 for the will of some 

 potent intrinsic afferent impulse or by some misdirection of volitional im- 

 pulses. 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 they attribute their 

 suddenly falling forward to the occurrence of the sensation that the ground 

 in front of them is suddenly sinking away beneath their feet. Without 

 trusting too closely to the interpretations the subjects of these disorders give 

 of their own feelings, and remembering what was said above concerning ver- 

 tigo, we may at least conclude that the unusual movements are in many 

 cases due to a disorder of the coordinating mechanism, brought about by 

 strange or disordered sensory impulses. And this view is supported by the 

 fact that many of these forced movements are accompanied by a peculiar 



