678 



PHYSIOLOGY 



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 

 t hem altogether. Any support given by the hand enables the animal 

 to rest quietly. After some months these disorders gradually dis- 

 appear, and the animal learns to guide its movements by sensa- 

 tions 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 pro- 

 duced if that part of the 

 brain which alone is educat- 

 able, viz. the cerebral cortex, 

 be excised. Extirpation of 

 the cerebral hemispheres in 

 pigeons causes no disorders 

 of equilibrium, but extirpa- 

 tion, after destruction of the 

 labyrinth, brings back the 

 disorders which were noted 



FIG. 312. Abnormal posture of pigeon, in during the first days after 

 i which the labyrinth had been extirpated on t ^ e operation, and these 

 one side five days previously. (EWALD.) r 



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. 312). Loss of tone 

 after complete destruction is well shown in the following experiment 

 devised by Ewald : 



A small lead bullet is hung up 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. 313). 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. 



