606 



PHYSIOLOGY 



those of his pupil Ewald, showed that these effects might last twelve to 

 eighteen months, or be permanent, and must therefore be regarded as 

 an Ausfallserscheinung, i.e. as due to abolition of a function and not to the 

 arousing of a function by abnormal stimulation. 



Most of the experiments on this subject have been carried out on pigeons 

 on account of the easy accessibility of their semicircular canals. Confirma- 

 tory observations have, however, been made on mammals. After destruc- 

 tion of all the canals or of the whole membranous labyrinth on both sides, 

 disturbances of equilibrium are aroused which may last for a considerable 

 time. The animal can neither stand, nor fly, nor maintain any fixed attitude, 

 but is constantly moving about incoherently and often so violently that 

 it is necessary to pad its cage in order to prevent it from injuring itself. 



Although the movements are so violent, 

 very little guidance suffices to stop them 

 altogether. Any support given by the 

 hand enables the animal to rest quietly. 

 After some months these disorders 

 gradually disappear, and the animal 

 learns to guide its movements by 

 sensations of touch and sight alone ; 

 but they are instantly brought back 

 in all their severity if the eyes be 

 bandaged, so as to deprive the co- 

 ordinating centres of the guiding visual 

 sensations. 



The same effect is produced if that 

 part of the brain which alone is edu- 

 catable, viz. the cerebral cortex, be 

 excised. Extirpation of the cerebral 

 hemispheres in pigeons causes no dis- 

 orders of equilibrium, but extirpation, after destruction of 'the labyrinth, 

 brings back the disorders which were noted during the first days after the 

 operation, and these disorders are now permanent. Recovery even in the 

 presence of the cerebral hemispheres is, however, never really complete. 

 Although the animal may be able to walk and fly very fairly, it suffers 

 from a loss of power and loss of tone which affect all its muscles, but 

 especially those moving the trunk and neck. If the labyrinth has been 

 extirpated only on one side, then this loss of tone is noticed chiefly on 

 the opposite or contralateral side of the body (Fig. 314). Loss of tone 

 after complete destruction is well shown in the following experiment 

 devised by Ewald : 



A small lead bullet is hung by a thread to the beak of the pigeon. As 

 the bird moves about the bullet swings, the head following its movements ; 

 finally the bullet happens to fall over the beak of the animal the head is now 

 found to be fixed in the position shown in the figure (Fig. 315). The anterior 

 muscles of the neck are too weak and toneless to restore the head to its 



FIG. 314. Abnormal posture of pigeon, 

 in which the labyrinth had been ex- 

 tirpated on one side] five days pre- 

 viously. (EWALD.) 



