THE CENTRAL NERVOUS SYSTEM 735 



When the head turns towards the anode the eyes move in the same 

 direction, and then undergo jerking movements towards the kathode. 

 There is at the same time a feeling of vertigo. Complex as such an 

 experiment is, involving as it does stimulation of so many structures 

 within the cranium, there is reason to believe that it is the excitation 

 of the semicircular canals, or their cerebellar connections, that is re- 

 sponsible for these forced movements. For when the experiment is 

 performed on a pigeon, forced movements are caused so long as the 

 membranous canals are intact, but not after they have been destroyed 

 (Ewald). The observation of Rawitz, that the peculiar rotatory 

 movements of the so-called Japanese dancing mice are associated 

 with marked anatomical peculiarities in the labyrinth, is a new fact in 

 favour of the connection of the canals with the maintenance of equili- 

 brium and the sense of rotation. So is the relation between the degree 

 of development of the canals in different species of birds and the degree 

 of agility in the co-ordination of their movements (Laudenbach). 



But forced movements may also follow injuries (especially uni- 

 lateral) to many portions of the brain e.g., the pons, crus cerebri, 

 posterior corpora quadrigemina, corpus striatum, even the cerebral 

 cortex, but above all the cerebellum. The movements are of the 

 most various kinds. The animal may run round and round in a circle 

 (circus movements) ; or, with the tip of its tail as centre and the 

 length of its body as radius, it may describe a circle with its head, as 

 the hand of a clock does (clock-hand movement) ; or it may rush 

 forward, turning endless somersaults as it goes. Intervals of rest alter- 

 nate with paroxysms of excitement, and the latter may be brought on 

 by stimulation. In man forced movements associated with vertigo 

 have been sometimes seen in cases of tumour of the cerebellum e.g., 

 involuntary rotation of the body in tumour of the middle peduncle. 

 No entirely satisfactory explanation of these forced movements has 

 been given. They are evidently connected with disturbance of the 

 mechanism of co-ordination, leading to a loss of proportion in the 

 amount of the motor discharge to muscles or groups of muscles 

 accustomed to act together in executing definite movements. For 

 instance, in circus movements the muscles of the outer side of the 

 body contract more powerfully than those of the inner side, and the 

 animal is therefore constrained to trace a circle instead of a straight 

 line, the excess of contraction on the outer side being analogous to 

 the acceleration along the radius in the case of a point moving in a 

 circle. 



Co-ordination of Movements. The capacity of executing some 

 co-ordinated movements, occasionally of considerable complexity, 

 seems to be inborn in man, and to a still greater extent in many of 

 the lower animals. The new-born child brings with it into the world 

 a certain endowment of co-ordinative powers ; it has inherited, for 

 example, from a long line of mammalian ancestors the power of 

 performing those movements of the cheeks, lips, and tongue, on 

 which sucking depends ; perhaps from a long, though somewhat 

 shadowy, race of arboreal ancestors the power of clinging with hands 

 and feet, and thus suspending itself in the air. Many movements, 



