nevertheless have a sense of position. He decided, 

 therefore, that the otoliths within the utricle and 

 saccule must be responsible for the static and posi- 

 tional sense. The mechanism of stimulation of the 

 receptors has been controversial. According to the 

 theory of Breuer, the gliding of the otoliths and bend- 

 ing of the hairs of the sensory cells caused by this 

 gliding during changes of the position of the head is 

 the stimulus. This theory has been rejected l)y later 

 workers (7, 78). The effective stimulus is now thought 

 to be the pull of gravity. The sensory cells will be 

 differentially stimulated in different positions of the 

 head since the otoliths will obey the law of gravity. 

 When the stimulation of the utricular maculae on 

 both sides is equalized, the sensation is that of normal 

 position, with the vertex of the head up and its base 

 down. Any disturbance of this equilibrium, as must 

 take place in a changed position of the head, neces- 

 sarily exerts a different pull of gravity upon the re- 

 ceptor structures. Experiments have demonstrated 

 that the utricle is the source of responses to gravity, 

 centrifugal force and linear acceleration (i, 63, 66, 

 67, 71, 94). By these various means of stimulation the 

 otoliths are made to change their relative orientation 

 with respect to the underlying macular surface. 

 Electrical responses recorded from the frog by Ross 

 (94) made it possible to distinguish between two types 

 of gravity receptors. One type responds when the 

 head is tilted out of the level position; the other type 

 signals only the return of the previously tilted head to 

 level. Cohen (15) describes four receptor types in the 

 lobster. Adrian (i) recorded the potentials appearing 

 in the vestibular nuclei of cats when the head is 

 tilted. In a lateral tilt, with the recording side lower- 

 most, the frequency of the discharge increased with 

 increasing tilting (fig. 7). In no case was there an 

 increase in frequency when the tilt was in the opposite 

 sense, i.e. raising the side under examination and 

 lowering the other. The frequency of the discharge 

 declined very slowly as an expression of a slow adapta- 

 tion of the receptors. It is interesting to note that the 

 responses of different stimuli (tilting, rotation) were 

 not found in the same parts of the vestibular nuclei. 

 This may well indicate some sort of functional locali- 

 zation within the nuclei. 



The utricle appears to be the organ of major im- 

 portance in postural reflexes and in the differential 

 distribution of muscular tone in the various laby- 

 rinthine reflexes. 



The function of the saccule is more obscure and 

 still imperfectly known. It can be destroyed on both 



VESTIBULAR MECHANISMS 557 



10 20 



Inclinaticm in degrees 



FIG. 7. Response of gravity receptors. Relation between tilt 

 of the head and frequency of discharge in units from several 

 animals. The degree of lateral tilt of the head is shown in the 

 upper inset. The impulses were recorded from the right side 

 while the head was being tilted to the right. [From Adrian (i).] 



sides without disturbing labyrinthine reflexes, even in 

 the rabbit, an animal in which these reflexes are highly 

 developed. It has been considered that the saccule is 

 not an essential part of the vestibular mechanism but 

 rather an organ associated with the cochlea and de- 

 signed for the perception of vibrational stimuli (6, 94, 

 123). Vibrations acting upon the mass of otoliths 

 should thus transmit corresponding oscillations of 

 pressure to the ciliate cells. More recent experiments 

 by Lowenstein & Roberts (67) upon elasmobranchs 

 have presented evidence that the fibers conducting 

 iinpulses in response to vibrational stimuli are derived 

 from the anterior two thirds of the saccular inacula 

 (and the papilla basilaris and macula neglecta). In 

 higher vertebrates the saccule has probably lost its 

 auditory function. It is unlikely that the sound vibra- 

 tions transmitted from the oval window to the peri- 

 lymph are further propagated in that part of the 

 labyrinth represented by otolith organs and the three 

 semicircular canals. The only exception may be the 

 effect of very violent explosive sounds. The wave of 

 pressure in the endolymph and perilymph set up by a 

 sudden, very loud sound may be sufficient to stimu- 

 late the receptor cells of the semicircular canals, the 

 utricle and the saccule. The subjective sensation is 

 then one of vertigo, or of a sudden displacement in 

 .space. The reflex response to such stimulation is a 

 sudden movement of the head, such as normally tends 



