1098 



HANDBOOK OF PHYSIOIOOV 



NEUROPHYSIOLOGY II 



that these reflexes are still present after removal of 

 the neck muscles when endings in the upper verte- 

 bral joints are still present. The neck muscles are not 

 thereby excluded from playing some part in these 

 reflexes; the small muscles of the neck are richly 

 supplied with muscle spindles (Cooper, personal 

 communication). In an experiment on man the head 

 was kept still and the trunk was moved; the corre- 

 sponding eye movement was small (67). 



NYST.AGMUS 



Although rotation of the head in the horizontal 

 plane to the right produces contraction of the left 

 lateral rectus, this contraction is not long maintained 

 but is succeeded by a quick swing of the eye to the 

 right, followed by deviation to the left at a rate 

 related to the rotation, another sharp flick to the 

 right, and so on. This is \estibular nystagmus. The 

 obvious suggestion is that the sharp flick is induced 

 by the approach of the eye to the limits of movement 

 and that this is signalled by the proprioceptors of the 

 eye muscles or by tissues of the orbit. This suggestion 

 can be excluded, de Kleijn injected a local anes- 

 thetic into the eye muscles and found no change in 

 the nystagmus (loi). McCouch & Adler (103) were 

 quite unable to modify a vestibular nystagmus by 

 pulling on any of the extraocular muscles. Only the 

 midbrain is necessary for nystagmus since it persists 

 after section of the brain at the level of the oculo- 

 motor nucleus (loi, 113). It seems, therefore, that 

 the sharp flick is initiated by midbrain structures 

 when the discharge in the motor nucleus has per- 

 sisted at a high level for a certain time. 



Vestibular nystagmus may also be produced by 

 unilateral injury to the labyrintli, the vestibular 

 nuclei or the cerebellum, and may be accompanied 

 by head nystagmus. Naturally other means of ex- 

 citing the labyrinth by caloric or galvanic stimuli 

 will also cause nystagmus as long as the labyrinth is 

 intact. 



There is no doubt of the existence of a projection 

 from the retina to the cerebellum (126, 144). Many 

 of the responses to pulling on extraocular muscles 

 which appeared after latencies up to 100 to 150 

 msec, were found in the superior cerebellar peduncle 

 (38) and appeared to be due to a pathway through 

 the cerebellum. Little is known as yet about possible 

 functions of such pathways. 



.JiDVERSIVE MOVEMENTS OF EYES 



In tlie lower vertebrates, movements of the two 

 e>es in response to a visual stimulus in the peripheral 

 field ma\' be independent. The extreme case is the 

 chameleon, the eyes of which perform constant in- 

 dependent scanning mosements but converge onto 

 an interesting target. 



In birds, mov-ement of the two eyes is usually con- 

 jugate; and in mammals all normal movements of 

 the two eyes are closely linked and consist of con- 

 jugate movements and of movements of convergence 

 and divergence. When a stimulus — particularly a 

 moving stimulus — falls on the peripheral retina, the 

 eyes and often the head are moved so as to allow the 

 image to fall on the region of greatest retinal sensi- 

 tivity. 



The greater the difference between acuity at the 

 fovea or area centralis and the rest of the retina the 

 greater the movements required to examine objects 

 in the visual field. Thus the rabbit is believed to have 

 fairly uniformly poor retinal sensitivity, and it makes 

 few head movements and few adversive eye move- 

 ments. The squirrel probably has very high sensi- 

 tivity in its pure cone retina and again makes few 

 head or eye movements. On the other hand, birds 

 make \ery extensive head movements to bring visual 

 images onto the fovea. 'Bird-like' movement of the 

 head in ordinary speech means rapid movement of 

 the head with pauses during which the head is held 

 quite still, presumably for fixation. On the whole 

 the larger birds show more eye movement, and the 

 pelican, which cannot move its enormous beak sud- 

 denly, has a considerable range of eye movement. 

 In those birds which do move their eyes freely, head 

 movement is slower and less jerky. The jackdaw 

 moves its eyes up to 30°, particularly, it is said, in 

 unfamiliar surroundings. Otherwise it moves both 

 head and eyes. The pigeon and hen both show ad- 

 versive movements of about 10°. In the owl the eyes 

 are tubular and fixed to the orbit. Here all movement 

 is carried out by the head (121). A good compara- 

 tiv-e account of these movements is given by Bartels 



(9)- 



Adversive movements have been studied by 



making the visual field move and watching the move- 

 ments of the head and eyes. As the field moves, the 

 head and eyes follow over a certain range, giving a 

 so-called pursuit movement. This is interrupted by a 

 quick flick of the head and eyes in the opposite direc- 

 tion, and the pursuit movements begin again. This 



