654 



PHYSIOLOGY 



as an Ausjallserscheinung, 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 move- 

 ments 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 de- 

 prive the co-ordinating centres of 

 the guiding visual sensations. 



The same effect is produced if 

 that part of the brain which alone is 

 educatable, viz. the cerebral cortex, 

 be excised. Extirpation of the cere- 

 bral hemispheres in pigeons causes 

 no disorders of equilibrium, but ex- 

 tirpation, after destruction of the 

 labyrinth, brings back the dis- 

 orders which were noted during 

 the first days after the operation, and these disorders are now per- 

 manent. 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. 325). 

 Loss of tone after complete destruction is well shown in the following experi- 

 ment 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. 326). The anterior 

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

 normal position against the weight of the bullet. No such phenomena are 

 presented by a normal bird. 



FIG. 325. Abnormal posture of pigeon, 

 in which the labyrinth had been ex- 

 tirpated on one side five days pre- 

 viously. (EWALD.) 



