170 PHYSIOLOGY 



muscle ; for a want of impression becomes a cause of action [a muscle, 

 antagonized by elastic tissue, ceasing to act, permits tbe elastic motion]. 

 In a gutta serena the iris is sometimes paralytic, and in others it is not, 

 which, when under certain circumstances, can be discovered ; and this I 

 shall now consider under the four following heads :- — 



First, a total loss of the susceptibility of impression of light in the 

 immediate organ of vision, in both eyes : second, such [loss] attended 

 with a paralysis of the muscles of the iris : third, a total loss of the sus- 

 ceptibility of the impression of light in the immediate organ of vision of 

 one eye : fourth, [such loss] attended with a paralysis of the muscles of 

 the iris of that eye. 



The first and second of these will produce no variety ; because, as the 

 stimulus of light upon the retina becomes the cause of action of the 

 sphincter of the iris, it will be impossible for the iris [in the absence of 

 that stimulus] to have any motion ; and therefore it cannot be deter- 

 mined whether the sphincters are paralytic or not. 



But the third admits of variety ; for if the muscle is not paralytic, 

 then we shall find that the stimulus of light upon the retina of the 

 sound eye becomes a stimulus to the sphincter of the iris of the diseased 

 eye; so that it will contract upon the light being thrown upon the 

 sound eye, but not so much as if its own retina could have been affected 

 by light. 



The fourth admits of no variety ; but it informs us whether the 

 sphincter of the iris is paralytic or not ; for if it does not contract 

 upon the light being thrown upon the sound eye, we may be sure it is 

 paralytic. 



From all which we may judge, or reasonably suppose, that the dila- 

 tation of the pupil arises from elasticity, and not from muscular con- 

 traction ; for its greatest dilatation is always attended with the greatest 

 paralysis of the other parts. 



"Whenever a person is totally blind in both eyes, we find that the 

 pupil is dilated to its full extent ; but when the blindness is only in one 

 eye, and [there be] no paralysis in the sphincter of the iris of the dis- 

 eased eye, we find that the pupil will contract if the light is thrown 

 on the sound eye. 



Dr. Eobertson's (of Kingston) case belongs to the [fourth condition]. 

 There is a total paralysis of the left eye, both of the retina and of the 

 sphincter ; so that his pupil is very much dilated, and shows no signs of 

 contraction when the light is thrown either upon the same eye or on 

 the sound one. He was % electrified, but it did him no good. 



It is a common observation, that people, as they grow old, grow longer- 

 sighted, ?'. e. the focal point appears gradually to move to a greater 

 distance ; but this is not the case. It is not a change in the position 



