804 THE EYE AND VISION [CH. LVI. 



eye, especially the crystalline lens, participate in their causation. The response of 

 the eye to non-luminous stimuli lasts some time, and is spoken of as a " blaze current " 

 An analogous response has been seen in skin, plant-tissues, etc. 



Gotch has studied the photo-electric changes in the frog's eyeball with the 

 capillary electrometer. He, like Waller, draws attention to the long latent period and 

 sustained character of the response. The photo-electric changes are all monophasic 

 effects, whether produced by illumination, or by shutting off the light. Gotch 

 suggests there are two chemical substances in the retina, one of which reacts to light, 

 the other to darkness. Each reaction is a change of the same type, but for the change 

 to occur markedly, the eye must be previously adapted, i.e., the substances must 

 undergo a phase of metabolism under conditions opposite to those which evoke the 

 reaction effects. Observations with red and green light do not support the view 

 that the photo-chemical changes are of opposite characters, for the photo-electric 

 change is always in the same direction, differing only in period of latency, that for 

 red being the longer. 



MOVEMENTS OF THE EYEBALL 



Protrusion of the eyeballs occurs (1) when the blood-vessels of 

 the orbit are congested ; (2) when contraction of the plain muscular 

 fibres of the capsule of Tenon takes place ; these are innervated by 

 the cervical sympathetic nerve; and (3) in the disease called 

 exophthalmic goitre. 



detraction occurs (1) when the lids are closed forcibly; (2) 

 when the blood-vessels of the orbit are comparatively empty; 

 (3) when the fat in the orbit is reduced in quantity, as during 

 starvation; and (4) on section or paralysis of the cervical sympa- 

 thetic nerves. 



The most important movements, however, are those produced by 

 the six ocular muscles. 



The internal rectus turns the eyeball inwards, the external rectus 

 turns it outwards. If the superior rectus acted alone, it would turn 

 the eyeball not only upwards, but owing to the sloping direction of 

 the muscle, the eyeball would be turned inwards also ; in turning 

 the eyeball directly upwards, this inward movement is arrested by 

 the outward tendency of the inferior oblique. Similarly, in turning 

 the eyeball directly downwards, the inferior rectus acts in conjunc- 

 tion with the superior oblique. Movements in intermediate directions 

 are produced by other combinations of the muscles. 



These muscles are all supplied by the third nerve except the 

 superior oblique, which is supplied by the fourth and the external 

 rectus by the sixth nerve. (See p. 640.) 



The muscles of the two eyes act simultaneously, so that images 

 of the objects looked at may fall on corresponding points of the 

 two retinae. The inner side of one retina corresponds to the 

 outer side of the other, so that any movement of one eye inwards 

 must be accompanied by a movement of the other eye outwards. 

 If one eyeball is forcibly fixed by pressing the finger against it so 

 that it cannot follow the movement of the other, the result is 



