PUPIL. 



found thai the pupil not only dilated and contracted itfi-lf, 

 wlicn one of the eyes was fliut, but ;dfo vvlien botii were 

 open. F. Paul found, from repeated experiments, that the 

 pupil not only of cats, but alfo of men, and other animals, 

 always contrafted itfelf when the eye was cxpofed to a 

 bnVht light, and a;Tain dilated itfelf when the light was 

 faint and languid. This he always obferved, as well when 

 both eyes were open, as when one of them was (liut. The 

 fame remark had been made by Achillinus, in a treatifc pub- 

 liftied in 1322 ; and the fatt was known to the Arabians, 

 Rliazes and Avjcenna. That the pupil of the eye is en- 

 larged, in order to view remote objefts, and that it is con- 

 Irafted while we are viewing thofe that are near, is a faft 

 with which Scheiner, in the beginning of tlic leventecnth 

 century, was well acquainted, and which he proved by ex- 

 periments, and ilkdtrated by figures. When a needle, or 

 ;iny fmall objcdt, is brought near to the eye of any perfon, 

 wlio looks attentively at it, the pupil, lie fays, is plainly 

 ieen to contraft ; and it as conltantly expands whenever it is 

 ivithdrawn. 



This alteration of the pupil is cfFefted by certain mufcu- 

 iar fibres on the outfide of the uvea, which arrive from 

 nerves detached hither from the fclerotica. Thefe fibres 

 proceeding ftraight from their origin towards the centre, 

 terminate in the orbicular limb or verge of the pupil, which 

 '5niifts of orbicular fibres, whereby the figure and fpace of 

 the pupil arc defined. The firtl, or longitudinal fibres, di- 

 late the aperture of the pupilla ; the latter, or orbicular 

 ones, conflringe it. 



Some authors, however, attribute the motions of the pu- 

 pilla to the ligamentum ciliare ; and others think, that both 

 this, and the fibres of the uvea, concur Iierein. Dr. Der- 

 ham adds, that while the pupil opens and fliuts, the liga- 

 mentum ciliare dilates or compreifcs the cryftalline, and 

 brings it nighcr to, or farther from, the retina, as the ob 

 jedl is more or lefs remote. 



There is no doubt that the change to which tlie pupil is 

 fubjeft is the cfFett of light upon the eye. Dr. Hartley, 

 Obf. on Man, vol. i. p. 219, fuppofes, that the light 

 which enters at the pupil has great efficacy in cont^-acling 

 both the greater and lefs rings of the iris, as may be con- 

 cluded, he fays, from the immobility of the pupil in a gutta 

 lerena ; alfo becaufe, on this fuppofition, t'le light which 

 paffes in at the pupil muft, by contraifling the lefs ring, be- 

 come a check and guard againll its own too free admiffion, 

 which is agreeable to the tenor of nature in like inllances. 

 The retina, he obferves, extends to the greater ring, and 

 may fend fome nervous fibres to it, and even to the iris. 

 Dr. Whytt alfo fays, Eflay on vital and involuntary Mo- 

 tion, p. 112, that the contradlion of tlie pupil is not per- 

 formed by the aftion of light upon the iris, but upon the 

 retina; fincc whatever intercepts tlie rays of light, i'o as to 

 prevent their reaching the retina, caufes a preternatural di- 

 latation of the pupil ; becaufe in a cataratf:, where the cryf- 

 talline humour, being confiderably opaque, intercepts a great 

 part of the rays, the pupil lofes a good deal of its contrac- 

 tile power. This author alfo obferves, that the widenefs of 

 the pupil in a fyncope, apoplexy, and confirmed gutta 

 ferena, fhews that, iii order to dilate the pupil to its largefl 

 iixij, no effort of the mind is neceffary, but only the fuperior 

 contraftile power of the longitudinal fibres of the uvea, 

 when its circular niufcle is not excited into aclion by the 

 (limulus of light on the retina. M. Mery informs us ( Mem. 

 Ac. Paris, for 1704) that, having plunged a cat in water, 

 and expofing her eye to the ftrong light of the fun, the 

 pupil was not at all contraCied by it ; whence he infers, 

 that the contraftion of the iris is not produced by the aftion 



of the light, but by fome other circumRance. For he con- 

 tends, that the eye in this fit nation receives more light tlia.i 

 in the open air. M. de la Hire, in reply, endeavours tc 

 fliew, (Mem. Ac. Par. for 1709) that fewer rays enter the 

 eye under water, and that, in thofe circumflances, it is nor 

 fo liable to be alfefted by them. Befides, it is obvious to 

 be remarked, that the cat muft be in great terror in tin.; 

 fituation ; and being an animal that has a very great vo- 

 luntary power over the mufcles of the iris, and being now 

 extremely attentive to every thing about her, fhe might have 

 her eyes open, notwithlfanding the adtion of the light upon 

 it, and though it might be very painful to her. 



The figure of the pupil, in various animals, is wonder. 

 fully adapted to their various circumftances and occafions 

 in fome, f- gr. in man, it is round, that form being fitted 

 for the pofition of our eyes, and the various ufes we make 

 of them in all direftions. 



In others, it is elliptical or oblong : in fome of which, 

 c. gr. the horfe, fheep, ox, &c. the elliplis is tranfverfe, and 

 the fifliire large, to enable them to fee laterally, and even 

 with a little light ; and thereby both to gather their food 

 the better in the night, and to avoid dangers on cither fide. 

 In others, e. gr. the cat, the ellipfis is ereft, and is alfo 

 capable of opening very wide, and fliutting very clofe ; by 

 means of the latter of which ftates, that animal can exclude 

 all, but, as it were, a fingle ray of light, and fo avoid all 

 the inconveniencies of the bright fuii ; and by the former, 

 it can take in all the fainteft rays, and thus avoid the in- 

 conveniencies of the night : an incomparable provifion for 

 thefe animals, which are to watcli and way-lay their prey 

 both by day and night, to fee upwards and downwards, to 

 climb, &c. See Eye and Vision. 



PuriL, Clofure of. A preternatural fmallnefs of the 

 pupil always renders the patient incapable of difcerning ob- 

 jefts well at night-time, and occafions a diminution of vifion. 

 The confequence of a complete clofure of the pupil, a cafe 

 termed fyiiize/is, is total blindncfs. Both thefe affeftions 

 only differ from each other in degree, and they originate 

 from the fame caufes. The molt frequent caufe is a violent 

 inflammation of the eves, particularly when fuch diforder 

 extends to the iris, and arifes at a period when the anterior 

 and pofterior chambers of the aqueous humour contain none 

 of this fluid. This cafe principally happens after the ex- 

 traftion of the cataract. Here the inflammation conftantly 

 affects the capfule of the cryftalline lens, fo as to make it 

 opaque, and at the fame time adhere behind the pupil to 

 the iris ; a circumftance which is well defcrving attention. 

 Not iinfrequently, however, a clofure of the pupil take< 

 place during ophthalmy, notwithttanding both chambers of 

 the eye are full of the aqueous humour. Wounds sf the 

 iris fometimes caufe this unpleafant occurrence, efpecially 

 when they continue open, and do not heal. It is obferved 

 to be mofl frequently produced by fuch wounds as divide 

 the radiated fibres of the iris tranfverfely. Thefe wounds, 

 however, do not invariably remain open : fometimes they 

 clofe, and then the pupil undergoes no alteration. Even 

 when fuch wounds do not unite, a clofure of the pupil does 

 not always follow. In thefe initances, the natural pupil has 

 been remarked to become clofed, and the preternatural aper- 

 ture in the iris to be widened, when the eye was cxpofed to 

 the light ; and, on the contrary, immediately the light was 

 diminifhed, the pupil became dilated, and the other opening 

 was contracted and clofed. - Wounds of the iris in the other 

 direftion, even wlien they remain difunited, often caufe little- 

 or no diminution of the natural motion of the pupil. 



It fometimes happens, that a greater or lefler portion of 



tlie iris is detached from the circumference of the cornea ; 



C 2 and, 



