620 COORDINATING MECHANISMS. [BOOK in. 



the plane on which it is placed, it falls down. The special coordi- 

 nating mechanism for balancing must therefore in this animal be 

 situated in the optic lobes ; but after removal of these organs, the 

 animal is still capable of a great variety of coordinate movements : 

 unlike a frog retaining its spinal cord only, it can swim and leap, 

 and when placed on its back immediately regains the normal posi- 

 tion. The cerebellum of the frog is so small, and in removing it 

 injury is so likely to be done to the underlying parts, that it becomes 

 difficult to say how much of the coordination apparent in a frog 

 possessing cerebellum and medulla is to be attributed to the former 

 or to the latter ; probably, however, the part played by the former 

 is small. In the mammal, as we have stated, removal of the whole 

 middle and hind brain does away with the most marked of these 

 coordinating mechanisms. Removal of the pons Varolii alone has 

 the same effect. Injury to, or disease of, the more superficial parts 

 of the corpora quadrigemina or of the cerebellum, does not appear 

 to influence the movements of the body at large to any striking 

 extent ; but there are many pathological cases, as well as experi- 

 mental observations, tending to associate the coordinating mechan- 

 isms of which we are speaking with the deeper parts of the cere- 

 bellum. It would be hazardous, in the present state of our know- 

 ledge, to make any definite statement concerning the share taken 

 by these several cerebral structures in the various coordinations. 



Forced Movements. 



All investigators who have performed experiments on the brain, 

 have observed as the result of injury to various parts of it remark- 

 able compulsory movements. One of the most common forms is 

 that in which the animal rolls incessantly round the longitudinal 

 axis of its own body. This is especially common after section of 

 one of the crura cerebri, more particularly of the external and 

 superior parts, or after unilateral section of the pons Varolii, but 

 has also been witnessed after injury to the medulla oblongata and 

 corpora quadrigemina. Sometimes the animal rotates towards and 

 sometimes away from the side operated on. Another form is that 

 in which the animal executes 'circus movements/ i.e. continually 

 moves round and round in a circle, sometimes towards and some- 

 times away from the injured side. This may be seen after several 

 of the above-mentioned operations, but is perhaps particularly com- 

 mon after injuries to the corpora striata and optic thalami. "There 

 is a variety of the circus movement said to occur frequently after 

 lesions of the nates, in which the animal moves in a circle, with the 

 longitudinal axis of its body as a radius, and the end of its tail for 



