1096 



HANDBOOK OF PHYSIOLORV 



NEUROPHYSIOLOGY II 



r^ 



fa) Xopf Meben 

 O-^tO 20 30 no 50 60 



fh> Kopf Heben 

 0—10 20 30 W SO 



10 20 30 to 50 60 7J 

 Kopf Senken 



10 20 30 W SO 60 70 

 kapf Senken 



FIG. 5. A: Relation of head movement about a binaural axis 

 to rotation of the eye about the visual axis in the rabbit. 

 Ordinates, rotation of the eye; abscissae, head movement down- 

 wards below, and upwards above; in both each scale division 

 represents 10°. The full line shows the effect of raising and lower- 

 ing the head; the broken line indicates the labyrinthine effect on 

 the eyes; thus, the shaded area represents the effect of the neck 

 reflexes on the eyes. B: .Same observations after the posterior 

 roots of Ci and C» have been cut. [From de Kleijn (50).] 



tude of lateral eye movement is not s;reater than 20° 

 to 30°. In the decerebrate cat, the vestibulo-ocular 

 reflex is remarkable as the only postural refle.x in- 

 volving the labyrinth which is executed at about its 

 normal speed. Rotation can e\oke action potentials 

 in the lateral rectus of up to 160 per sec. (117). The 

 latency between the onset of mo\enient and the first 

 action potential can be as short as i o to 20 msec. 

 Further study of this point is made difficult by the 

 latency of the vestibular endings to rotation. Accord- 

 ing to VVendt (143) the latent period in man for the 

 slow movement of vestibular nystagmus is about 50 

 msec, varying with acceleration from 40 to 80 msec. 

 For rotation in the horizontal plane, at about 30° 

 per sec, about 60 per cent of the rotational move- 

 ment is compensated. .Similar extent of compensation 

 is seen with slower movement. If a fixation point is 

 provided, about 80 per cent of 65° head movement 

 is compensated (143). 



By injecting fluid into one semicircular canal at a 

 time in the unanesthetized cat, .Szentagothai (131) 

 has obtained some evidence that each canal is linked 

 chiefly to two eye muscles. For example the left 

 lateral canal chiefly excites the left medial rectus 

 and the opposite lateral rectus. He has anatomical 



evidence that these actions are mediated by three 

 neuron arc connections through the medial longi- 

 tudinal Ijundle. In addition there are more gener- 

 alized excitatory and inhibitory connections from 

 each canal to the other muscles, mediated by multi- 

 synaptic connections in the reticular formation. 

 Earlier work b\' Lorente de No (gS) also showed 

 the connections with shortest latency to run in the 

 medial longitudinal liundle, but the animals were 

 anesthetized and the labyrinth intact and the im- 

 portance of the multisynaptic connections seemed to 

 be very much greater than the connections through 

 the medial longitudinal bundle. 



There is no maintained static labyrinthine reflex 

 response to rotation of the head in a horizontal plane. 

 This is attributed to the fact that no otolith organ is 

 excited by deviations of the head in this plane. De- 

 viations of 17° or more have been produced in the 

 rabbit i)y a static neck reflex (51). 



Reactions In I'prtical Movement 



Tilting of the head in the fore and aft plane (about 

 a bitemporal axis) excites all four vertical canals and 

 produces rotation about the visual axes in animals 

 with laterally directed eyes, in fish (10), the rabbit 

 (50) and the pigeon (11). In the rabbit this reac- 

 tion consists of a rapid component and a very stable 

 static component. According to de Kleijn (50) the 

 horizontal meridian can be kept accurately hori- 

 zontal while the raijiait head is mo\ed through 100°, 

 as shown in figure 5. This compensation is slightly 

 reduced by fixation of the neck or removal of the 

 cervical ner\es. A similar static reaction is seen in 

 fish. Here the compensation of movements of over 30° 

 from the position of rest is not more than three 

 quarters of the head mosement. According to the 

 graphs given b\' Benjamins (10), however, compensa- 

 tion may be perfect for the first 22.5° abo\e and 

 below the horizontal; he makes no comment on this 

 point in the text. 



Corresponding experiments on the pigeon gave 

 much smaller compensation for steady displacement, 

 the average figure being ^j.) of the angle of tilt of 

 the head (lo). This work has recently been repeated 

 by Sommer & W'hitteridge (127) as a result of a 

 recent paper by Merton (107) on man. The static 

 rotation of the eye in the pigeon is in fact only a 

 small fraction of the head rotation, Imt during rota- 

 tion at moderate speed the eye movement in the 

 decerebrate pigeon compensates for 60 to 70 per cent 

 of the head movement over a range of 10°. After 



