522 



Comforative Animal Physiology 



the head is not oriented but falls into an abnormal position due to the 

 force of gravity. If this animal is now placed on its side on the table, 

 the head will right itself. If a weighted board is placed on the upper side 

 of the animal, the head is not righted, indicating that unsymmetrical stimu- 

 lation of cutaneous receptors affords a sensory cue for righting. The 

 righting of the head in such labyrinthectomized rabbits is followed by 

 righting of the body. If the head is held so that it cannot be oriented, 

 the body of the rabbit will right itself, followed by righting of the head, 

 if permitted. This indicates that the neck may also be the source of sensory 

 cues for righting. That this is so can be demonstrated clearly in labyrin- 

 thectomized decerebrate (brain stem divided just anterior to entry of the 



Fig. 192. Relation between deflection of cupula in the intact semicircular canal of the 

 pike and time, during prolonged rotation. Solid lines indicate the position of the cupula 

 (measured in arbitrary units) after the onset, during and after rotation. The lower broken 

 lines indicate the deflection upon stopping the rotation at the times indicated. The upper 

 broken line indicates the theoretical movement of endolymph in the canal. Each division 

 of the time base is equal to 1-1/3 seconds. From Steinhausen.** 



eighth cranial nerve) animals. Forced rotation of the head in such ani- 

 mals produces an increase of extensor tonus (tonic contractions of ex- 

 tensor muscles are increased) on the side toward which the nose is turned. 

 Section of the cervical dorsal roots abolishes this response, suggesting that 

 sense organs (proprioceptors) in the neck muscles are its source. 



In normal vertebrates, distinct reactions to rotation are evident. When 

 an animal is rotated to the right, the eyes tend to remain fixed on an object 

 as long as possible, the head gradually moving to the left as far as possible, 

 then shifting suddenly to the right. This cycle of movement is repeated, 

 and is called head nystagmus. After rotation has been stopped, the nystagmus 

 is in the opposite direction. The eyes exhibit reactions similar to those of 

 the head, and these are referred to as eye nystagmus. Such compensatory 



