SQUINTING. 



the refult of habit and experience, by which the mind learns 

 to correa the impreifions of the fenfes. (See Edm. Medi- 

 cal Effays, vol. iii. J 12.) In order to afcertain the nature 

 of this afFedion, it wiU be neceffary to inveftigate its vane- 

 ties and caufes. , . 



Dr. Cullen was of opinion, that all the varieties of Itra- 

 bifmus, delcribed by Sauvages and others, might be included 

 under three fpecies, which he defignated by the appellations 

 habitualis, commodus, and necefarius. TUefrJl of thefe may 

 arife from the habit of ufiiig only one eye in viewing objefts, 

 while tiiere is no dcfeft either in the organ itfelf, or in the 

 mufcles which move it. In this way children may learn to 

 fquint from having objefts, which attraft their attention, 

 placed obhquely near the cradle ; in which cafe it will be 

 more eafy to view them with the oppofite eye alone, than to 

 direft both towards the fame point, and they will thus ac- 

 quire the habit of neglefting the ufe and motions of the 

 other eye. In a later period of life, the power of fquinting 

 voluntarily appears to be acquired by fome by this praftice 

 of ullng one eye. Thus perfons who look much through 

 telefcopes, and of courfe employ one eye almolt exclufively, 

 may acquire the power of moving the one more than the 

 other, or lofe a part of their command over the mufcles 

 of the negledled eye. 



Dr. Darwin mentions a fingular cafe of fquinting, in 

 which the patient was equally expert in the ufe of either 

 eye, but viewed every objeA prefented to him with only one 

 eye at a time, and always with the eye on the fide oppofite 

 the objeft. Thus, if the objeft was prefented on his right 

 fide, he viewed it with his left eye ; and when it was prefented 

 on his left fide, he viewed it vi?ith his right eye. At the fame 

 time, Dr. Darwin found that he turned the pupil of that 

 eye which was on the fame fide with the objett in fuch a 

 direftion, that the image of the objeft might fall on that 

 part of the bottom of the eye where the optic nerve enters 

 it, and where it would of courfe excite no impreflion ; and 

 this infenfible portion of the retina, Dr. Darvdn afcertained 

 by fome ingenious experiments to be four times greater in 

 this patient than in ordinary perfons. When an objeft was 

 held direftly before this patient, he turned his head a little 

 to one fide, and obferved it with but one eye, •viz. with that 

 moft diftant from the objeft, turning away the other in the 

 manner juft mentioned ; and when he became tired with ex- 

 jmining it with that eye, he turned his head the contrary 

 way, and obferved it with the other eye alone, with equal 

 facility ; but never turned the axes of both eyes on it at the 

 fame time. Phil. Tranf. vol. Ixviii. p. 86. 



The fecond fpecies of fquinting, however, which Dr. 

 Cullen ityles commodus, and which is the effedl of fome im- 

 perfedion in one of the eyes, is the mcft common form of 

 the difeafe. It has generally been admitted, or afcertained 

 by experiment, in thofe who have a confirmed fquint of this 

 nature, that one of the eyes is too imperfcdl to fee dif- 

 tinftly. Of this, however, the patient is not always con- 

 fcious, as was evinced in a young lady, whofe cafe is related 

 by fir Everard Home. Neither herfelf nor her friends be- 

 beved that any defeiS of the eye exilted ; and upon being 

 aflted if (he faw objeds diilinftly with both eyes, file faid 

 certainly, but that one was ftronger than the other. To 

 afcertain the truth of this, he covered the ilrong eye, and 

 jrave her a book to read, when, to her allonifhmeiit, (he 

 found Ihe could not dilfinguifh a letter, or any other near 

 objed. More diltant objects (he could fee, but not dif- 

 tinftly : when (lie looked at a bunch of fmall keys in the 

 door of a book-cafe, about twelve feet from her, (he could 

 fee the bunch of keys, but could not tell how many there 

 were. 



The obfcurity of vifion in one eye, then, is the caufe of 

 this common fpecies of fquinting, and may occafion this ir- 

 regularity in the following way. The obfcure image, being 

 fo imperfeAly formed in the weak eye, as to excite little at- 

 tention in the mind, the ufe of the eye, and its uniform di- 

 reftion to the fame objeft with the other, may have been 

 neglefted from the beginning ; for as diftinft vifion was ob- 

 tained at once by the perfeft eye, the end was anfwered, and 

 therefore there was no neceffity for any exertion to employ 

 the other. Or on the other hand, in the effort to get rid 

 of the confufed image, the mufcles may have acquired an 

 irregular and unnatural aftion. It is remarked by fir E. 

 Home, that the direftion, which the eye takes under either 

 of thefe circumftances, is inwards, towards the nofe ; becaufe 

 the adduBor mufcle is ftronger, fhorter, and its courfe more 

 in a ftraight line, than any of the other mufcles of the eye. 

 Phil. Tranf. 



In the third fpecies, or flrabifmus necejfarius of Cullen, 

 the fquinting is the refult of fome peculiarity or change in 

 the form or fitualion of the eye or its parts ; whence a per- 

 feft and diilinft image of any objeft can only be obtained by 

 an oblique direftion of the axis of the eye. Dr. Porterfield 

 has pointed out two varieties of this kind, arifing, the one 

 from an oblique pofition of the cryftalline lens within the 

 eye, by which the image of an external objeft is refrafted 

 out of the line of the axis of the eye ; — and the other, from 

 an oblique pofition and greater protuberancy of the cornea, 

 which produces a fimilar effeft. (See Edin. Medical E(rays, 

 vol. iii. art. 12.) Thefe appear to be fynonimoas with the 

 Urabifmus a cryjlalUm, and S. myopum of Sauvages (Nofol. 

 Meth. clafs iv. gen. i. fpec. 12 & 9.) Other partial dif- 

 eafes of the eye may contribute to render obhque vifion the 

 moft convenient ; fuch as kucoma, hypopion, pterygium, &c. 

 enumerated by Sauvages as conftituting his eleventh fpecies, 

 S. caligantium. 



Cure of Squinting. — The two firft fpecies of fquinting, 

 arifing from the habit of ufing one eye only, or from the 

 weaknefs or imperfeftion of the other, may be foraetimes 

 cured. The third, which arifes from fome malformation of 

 the eye or its parts, is fcarcely remediable. 



The principle of cure in the two firft fpecies is nearly the 

 fame ; namely, the conftant exercife of the neglefted eye, 

 whether naturally weak or not. This may be effefted by 

 covering the ftrong eye, or that which is always employed, 

 and confining the perfon to the ufe of the neglefted or weak 

 eye. For in this way, the mufcles of the latter, from con- 

 ftant aftion, will become perfeft in the habit of direfting 

 the eye upon the objeft, gain ftrength in that aftion, and 

 acquire a power of adjufting the eye. When this is efta- 

 bli(hed in a fufhcient degree, the other eye may be fet at 

 liberty. The time that will be neceftary for the cure, will 

 depend upon the inveteracy of the habit, the length of time 

 that the mufcles have been left to themfelves, and the degree 

 of weaknefs of the fight ; for it is with difficulty that 

 mufcles acquire an increafed degree of aftion after having 

 been long habituated to a more limited contraftion. Dr. 

 Darwin obferves, that if the fquinting has not been con- 

 firmed by long habit, and one eye be not much worfe than 

 the other, a piece of gauze ftretched on a circle of whale- 

 bone, to cover the belt eye in fuch a manner as to reduce 

 the diftinftnefs of vifion of this eye to a fimilar degree of 

 imperfeftion with the other, (hould be worn fome hours every 

 day. 



For the cure of the curious cafe related by the fame in» 

 genious phyfician, in which there was no defeft in either eye, 

 but merely a depraved habit of ufing both eyes feparately. 

 Dr. Darwin fays, " a gnomon of thin brafs was made to 



ftand 



