734 A MANUAL OF PHYSIOLOGY 



some centre, to which afferent impulses pass from the internal ear 

 by the vestibular branch of the auditory nerve, and from which 

 efferent impulses pass out to the muscles. If, as there is strong 

 reason to believe, this centre is situated in the cerebellum, the 

 efferent path is in all probability an indirect one (perhaps by com- 

 missural fibres to the Rolandic area, and then out along the pyramidal 

 tract). Ewald has made a curious observation which illustrates the 

 peculiar relation of the semicircular canals to the muscular system, 

 namely, that the labyrinth (in rabbits) influences the course ot rigor 

 mortis in the striped muscles. Rigor does not come on so soon on 

 the side from which the labyrinth has been removed. 



It is the middle lobe of the cerebellum which seems to be concerned 

 in the co-ordination of movements and maintenance of equilibrium. 

 In birds and lower vertebrates the worm is alone present. The 

 cerebellar hemispheres become more prominent the higher we 

 ascend, and it cannot be doubted that they have important functions, 

 but what these are is entirely unknown. The fact that they are con- 

 nected chiefly with those parts of the cerebral cortex which are sup- 

 posed to be concerned in psychical and sensory processes suggests 

 that, at any rate, the superficial grey matter of the cerebellum is not 

 motor, and no movements can be obtained on stimulating it ; while 

 stimulation of the worm may cause movements of the eye. Excita- 

 tion of the line of junction of the superior worm with the lateral lobe 

 in animals which exhibit tonic contraction of extensor muscles after 

 excision of the cerebral hemispheres (acerebral tonus, as it is called) 

 causes relaxation of the extensors accompanied by contraction of the 

 antagonistic flexors for example, relaxation of the triceps and con- 

 traction of the biceps (Horsley and Lowenthal). But this can 

 scarcely be considered a reaction specific to the cerebellum. For 

 Sherrington, who finds that the tonus or spasm is largely due to 

 centripetal impulses coming from the rigid limb, has been able to 

 inhibit it by stimulation of various regions, among others the motor 

 cortex around the fissure of Rolando. 



Forced Movements. We have incidentally mentioned that in fishes 

 injuries to the semicircular canals may give rise to movements which 

 seem to be beyond the control of the animal, and which have 

 consequently received the name of 'forced movements.' It may be 

 added that when the internal ear of a Menobranchus (one of the 

 tailed amphibia) is destroyed on one side, rapid movements of rota- 

 tion around a longitudinal axis are observed. The animal spins 

 round and round apparently without voluntary control, purpose, or 

 fatigue. The direction of rotation is towards the side of the lesion, 

 the observer being supposed to look down upon the animal as it lies 

 in its normal position. After a time it becomes quiescent ; but the 

 forced increments can be again produced by pinching or exciting it 

 in other ways. In man, too, during the passage of a galvanic current 

 through the head by electrodes applied just behind the ears, a 

 tendency to move the head towards the anode is experienced. The 

 person may resist the tendency, but if the current be strong enough 

 his resistance will be overcome ; he will execute a forced movement. 



