CH. LVIIL.] MOVEMENTS OF THE EYEBALLS 849 



is the same as in extra-foveal vision of the normal eye ; in many of these cases there 

 is a central scotonia (blind spot), that is, the rodless fovea is blind ; there is reduced 

 acuity of vision as in the " dark-adapted " eye, and photophobia (fear of strong light); 

 nystagmus (oscillating movements of the eye) also occurs, due to absence of an area of 

 distinct vision. We are thus in typical cases of achromatopsia dealing with cases 

 of cone blindness. In nyctalopia (night blindness), on the other hand, we meet the 

 converse condition. Here there is an abnormal slowness of " dark adaptation," and 

 a pathological change known as retinitis pigmentosa is present, suggesting an im- 

 paired function of the visual purple. Pilocarpine has been found an effective drug 

 in such cases, and this is also interesting because it hastens the regeneration of visual 

 purple in the extirpated eye. 



The electrical variations in the retina under the influence of light were 

 discovered by M'Kendrick and Dewar, and have been recently reinvestigated by 

 Waller. The excised eyeball of a frog is led off by non-polarisable electrodes to a 

 galvanometer. One electrode is placed on the front, the other on the back of the 

 eye. If the eyeball is quite fresh, a current is observed passing through the 

 eyeball from back to front. When light falls on the eye this current is increased ; 

 on shutting off the light there is a momentary further increase, and then the current 

 slowly returns back to its previous condition. Waller explains this by supposing 

 that anabolic changes in the eye predominate during stimulation by light. With 

 the onset of darkness, the katabolic changes cease at once, and the anabolic more 

 slowly ; hence a further positive variation. 



As already stated, the current in a fresh eyeball passes from back to front before 

 the stimulus is applied, but this cannot be regarded as a true current of rest, but as 

 a current due to previous action which very slowly subsides. When this has 

 subsided, the true current of rest is from cornea to fundus, i.e., it is like that 

 of the skin, ingoing the response to stimulation is like that of the skin, out- 

 going. Waller has also studied the electrical responses of the eyeball to other 

 methods of stimulation ; if 'electrical currents are employed, and the eyeball is still 

 healthy, the response is always an outgoing current, whatever may be the direction 

 of the electrical current used as the stimulus. These currents of action are no doubt 

 mainly of retinal origin, but later Waller showed that the anterior portions of the 

 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. 



Retraction 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 



3" 



