1018 FORCED MOVEMENTS. [BOOK m. 



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 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 progres- 

 sion 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 powerful 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 movements are 

 more complicated 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 forwards or back- 

 wards. 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 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 passing from one part of the brain 

 to another, partly by generating new unusual 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 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 



