PHYSIOLOGY. 





eye depends on the state of the pupil ; 

 which is again influenced by the motions 

 of the iris. When, after shutting the eye- 

 lids, they are suddenly opened in a strong 

 Ikht, a disagreeable impression takes 

 place on the eye, and the iris dilates or 

 becomes broader: hence the pupil is 

 contracted, and the quantity of light ad- 

 iijittt'd into the eye diminished. An op- 

 posite change takes place when we go 

 from a strong into a weak light. These 

 motions depend entirely on the mode in 

 which light affects the retina ; for the iris 

 is of itself insensible to luminous rays. 

 The painful effect produced on the reti- 

 na by a strong light is obviated by the 

 contracted state of the pupil ; while the 

 opposite condition of that opening, in 

 darkness, is designed to admit a suffici- 

 ent quantity of rays, to produce a proper 

 impression on the retina. 



The seeing of bodies erect, although 

 their images are painted inverted on the 

 retina, is thus explained. An object is 

 .said to be inverted in respect to others 

 which are erect : now all objects what- 

 evefwe painted inverted on the retina, 

 and all therefore correspond to each 

 other in situation and connexion, just as 

 if they had been represented in their 

 natural position. All confusion is, there- 

 fore, guarded against in the mind ; to 

 which the image itself on the retina is not 

 communicated, but only an impression 

 caused by its formation. 



The motion of the iris contributes to 

 distinctness of vision by regulating the 

 quantity of light admitted into the eye : 

 and there is another provision tending to 

 the same effect; viz. the absorption of any 

 superfluous rays, which may have en- 

 tered the eye by the black covering of 

 the choroid coat. The utility of this 

 dark pigment may be understood from 

 observing the effects of its absence in the 

 albino, where it causes a tenderness of 

 the organ, and an impatience of the light. 

 Distinct vision also requires that the 

 focus of the refracted rays should fall ex- 

 actly on the retina, without falling short 

 of it in the vitreous humour, or being 

 elongated beyond it. The former fault 

 constitutes near-sightedness (myopia) ; 

 where the cornea and lens are too con- 

 vex, and the sight of remote objects is 

 imperfect. The latter defect is far-sight- 

 edness (presbyopia, as being common in 

 old persons) ; where an opposite condi- 

 tion of the eye obtains, and near objects 

 are seen imperfectly. 



As the eye possesses a very considera- 

 ble range of power in seeing distinctly 



both near and distant objects, it must 

 possess some powers of accommodation 

 adapting it to these differences of dis- 

 tance. Various opinions have been held 

 on this subject; but none are supported 

 by sufficiently direct and convincing ar- 

 guments to command universal assent. 

 The changes in the condition of the pupil 

 have some effect: it contracts when we 

 look at a near object, to exclude those 

 rays which would be too divergent for the 

 powers of the eye; and it dilates in the 

 opposite case, to take in the divergent 

 rays. Besides this, different physiologists 

 have admitted a power of motion in the 

 crystalline, by which its convexity may 

 be altered; a movement of that body back- 

 wards and forwards, in the eye, by the 

 ciliary processes, so as to place it at dif- 

 ferent distances from the retina; a com- 

 pression of the globe by the four recti 

 muscles, and consequent elongation and 

 shortening of the optical axis. 



That the retina, in the very axis of the 

 eye, is insensible, owing to the entrance 

 of an artery at that part, is shewn by ex- 

 periments, in which objects vanish when 

 their images are brought on that point. 



Single vision, with two eyes, probably 

 arises from habit ; for children seem to 

 see double ; and the same affection, after 

 diseases, lias been conquered by use. 



The sight would lead us into many er- 

 rors concerning the distance and figure 

 of objects, were it not corrected and 

 assisted by the touch. The person born 

 blind, mentioned by Cheselden, thought 

 when he had gained his sight, that all ob- 

 jects touched his eyes. A square tower, 

 at a great distance, appears round ; and 

 lofty trees, in a distant perspective, seem 

 no larger than small bushes that are 

 nearer to us. 



Hearing. The undulations of the at- 

 mosphere, excited by the vibrations of 

 sonorous bodies, are collected in the ex- 

 ternal ear and auditory passage, as in a 

 hearing trumpet, and are conveyed to 

 the membrana tympani, which they cause 

 to vibrate. The effect is transmitted 

 through the small bones to the watery 

 fluid that fills the internal ear, in which 

 the delicate filaments of the auditory 

 nerve float ; and by this nerve the sensa- 

 tion is conveyed to the brain. Muscles 

 attached to the small bones of the tym- 

 panum have the power of stretching or 

 relaxing the membrane; and probably 

 thereby adapt the organ to various quan- 

 tities of sound, by diminishing acute, and 

 augmenting the force of grave sounds, a* 

 the changes in the pupil of the eye ac- 



