736 FORCED MOVEMENTS. [BOOK in. 



exhaustion passes off the animal may remain for some little time 

 quiet, but some stimulus, intrinsic or extrinsic, 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 pro- 

 gression 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 muscles 

 of one side (the left for instance) of the body are more power- 

 ful than the other, and in consequence the body is continually 

 thrust towards 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 movement^ are 

 more complicated in their nature. No mere blocking of voli- 

 tional impulses will explain why an animal whenever it attempts 

 to move rolls rapidly over, or rushes irresistibly forwards or 

 backwards. It is not possible with our present knowledge to 

 explain how each particular kind of movement is brought about ; 

 and indeed the several kinds are probably brought about in dif- 

 ferent 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 passing from 

 one part of the brain to another, partly by generating new unu- 

 sual impulses, seriously affect the due working of the general 

 coordinating machinery. 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 

 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 impulses. Persons who have experi- 

 enced 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 



