244 SENSE OF SIGHT. 



The iris contracts or dilates according to the intensity of the light 

 that strikes the eye. If the light from an object be feeble, the pupil 

 is dilated to admit more of the luminous rays : on the contrary, if the 

 light be powerful, it contracts. We see this very manifestly on open- 

 ing the eyes, after they have been for some time closed, and bringing 

 a candle suddenly near them. It is one of the means frequently em- 

 ployed in cerebral disease to judge of the degree of insensibility. 



We shall presently inquire into the effect of contraction or dilatation 

 of the pupil on distinct vision ; and show, that they are actions for ac- 

 commodating the eye to vision at different distances. 



We may conclude, then, that the iris is one of the most important 

 parts of the visual apparatus ; that its functions are multiple : that 

 it is partly the cause of the achromatism of the organ, by preventing 

 the rays of greatest divergence from falling near the marginal parts 

 of the crystalline; that it corrects the aberration of sphericity; regu- 

 lates the quantity of light admitted through the pupil, and accommodates 

 the eye, to a certain extent, to vision at different distances. 



An enumeration of the multiform sentiments regarding the functions 

 of the ciliary processes, will show how little we know, that is precise, 

 on this matter also. They have often been considered contractile; 

 some believing them connected with the motions of the iris, others to 

 vary the distance of the crystalline from the retina. Jacobson 1 makes 

 them dilate the apertures, which he conceives to exist in the canal 

 godronne, so as to cause the admission of a portion of the aqueous 

 humour into the canal ; and thus to change the situation of the crys- 

 talline. Others believe, that they secrete the pigmentum nigrum; and 

 others the aqueous humour. But the processes are wanting in ani- 

 mals, in which the humours, notwithstanding, exist ; and in our igno- 

 rance of their precise function, it has been considered that there is no 

 opinion, perhaps, more probable than that of Haller ; 2 that they are 

 destined to assist mechanically in the constitution of the eye ; and have 

 no farther use. 



The function of the retina remains to be considered. It is the part 

 that receives the impression from the luminous rays, which impression 

 is conveyed by the optic nerve to the brain. It was, at one time, 

 universally believed to be the most delicately sensible membrane of 

 the frame. It has been shown by the experiments of M. Magendie, 3 

 that the sensibility of both it and the optic nerve is almost entirely 

 special, and limited to the appreciation of light; that the general 

 sensibility is exclusively possessed by the fifth encephalic pair ; and 

 that the nerve of special sensibility is incapable of executing its func- 

 tions, unless that of general sensibility is in a state of integrity. That 

 distinguished physiologist found, when a couching needle was passed 

 into the eye at its posterior part, that the retina might be punctured 

 and lacerated without the animal exhibiting evidences of pain. The 

 same result attended his experiments on the onj;ic nerves. These 

 nerves, both anterior and posterior to their decussation, as well as the 

 thalami nervorum opticorum, the superficial layer of the tubercula 



i Magendie, Precis, edit, cit, i. 78. 2 Element. Physiol., xvi. 4, 20. 



Op. cit., i. 83. 



