82 VERTEBRATE PHOTORECEPTORS 



tropic changes occur in the excised eye, it became apparent 

 that the faihire, after optic nerve section, is due to some 

 mechanism either in the eye muscles or in the blood vessels 

 which normally exert an inhibitory action on the retinal 

 movements. This was found to be the case, for when all 

 the eye muscles and blood vessels of a dark-adapted catfish 

 were cut, allowing only the optic nerve to be intact, typical 

 responses to light ensued. It was shown further that after 

 section of the superior oblique and the external rectus 

 muscles (those innervated by the trochlea and abducens 

 nerves) no pigment migration occurred. However, when 

 the remaining eye muscles were severed also (those inner- 

 vated by the oculo-motor nerve) pigment migration ensued 

 as in a normal animal. It became apparent then that the 

 inhibitory mechanism exists in association with these 

 muscles (or their blood vessels) which are supplied by the 

 III nerve. It was shown further that under these conditions, 

 electrical stimulation of the peripheral stump of the optic 

 nerve can overcome this inhibition. Hence experimental 

 evidence was produced offering physiological proof for the 

 existence of functional efferent fibers in the optic nerve. By 

 the balanced interaction of these fibers with a second extrinsic 

 set of nerve fibers (possibly ciliary nerves), which exert an 

 inhibitory influence upon the retinal elements, the normal 

 phototropic movements of the retinal elements are brought 

 about. In summing up Arey says, ''It is probable that 

 efferent impulses in the optic nerve fibers do not directly 

 stimulate the retinal elements to motion, but rather such 

 impulses have an indirect action, possibly by counteracting, 

 that is, blocking, the tonic inhibition exerted by the second 

 system of nerve fibers. If these efferent optic nerve fibers 

 fit at all into the scheme of 'nerve components', they may be 

 designated as visceral efferent elements." 



This inhibitory mechanism was not met with in Abramis 

 and Fundulus. Apparently, it is lacking also in the frog 

 eye. It is possible, therefore, that it may be peculiar to 

 Ameiurus alone. 



