On thi Impcifeniini rf Vifnn. 545 



as ufual, but not the extreme degrees without confidcrable pain. As thefe fymjrfdms have- 

 not been accounted for before in this way, the author adduces fome curious inftances oF 

 limilar afFeftions of other mufcles, and concludes that bad efFefts muft neceffiirily arifer 

 from every thing which irritates or weakens the parts themfelves, or the general habit ; 

 and that fuch means ought to be adopted as may foothe the parts in their fenfations, and 

 ^uiet and ftrengthen their aftions, fince in that vi-ay only the mufcular fibres can pofTtbly 

 recover their tone. 



On tlie fubjeft of double vifion the author remarks, that many opinions have been ad* 

 yanced to account for the fingle appearance of obj>£ls contemplated by both eyes. The 

 opinion of Dr. Reid of Glafgow, that vifion is fingle when the impreffions are made oji 

 correfpondent parts of the retina in tlie two eyes, and double when this is not the cafe, 

 appears to him to be ftrongly confirmed by the facts. This want of correfpondence may 

 be produced by fome change in the refrafting media of one of the eyes, or elfe by a want 

 of fimilar a£lions in the mufcles of both eyes refpe(ftively. The former takes place after 

 the cryflalline lens of one or both eyes has been extrafted, and the convex lens made ufe 

 of to produce the requifite focal adjuftment is not duly placed. The latter is more parti- 

 cularly the objeft of medical treatment. Mr. Home advifes repofe; that is to fay, that the 

 difordered eye fhould be covered for a time. For, as he remarks, the firft object of atten- 

 tion, with regard to {trained or over-fatigued mufcles, muft be to put them into an eafy 

 ftate, and confine them from motion ; and this practice is no lefs applicable to the mufcles 

 of the eye than thofe of other parts. 



As double vifion is produced by a moderate derlangement of the optic axes, fquinting is 

 effedted by a much greater derangement. In the cafe of fquinting, the author proves that 

 the obje£t is not feen by botli eyes ; but tliat one eye, more or lefs perfe£t, is directed to 

 the obje£t, while the other, which in fuch cafes is imperfect, is drawn afide by habit, in 

 order that its operation may not difturb ihe perception received by the other eye. T he 

 greater ftrcngth, fliortnefs and (traitnefs of the adductor mufcle caufcs the deviation to be 

 made towards the nofe. Thefe do£lrines .are illuftrated by appofite cafes. Squinting takes 

 place in three different circumftances ; that is to fay, where one eye has only an inditlintt 

 vifion ; where both eyes are capable of feeing objcdts, but the one is lefs pcrfe£t than the 

 other ; and where the mufcles of one eye have, from praflice, as in the cafe of frequently 

 looking through telefcopes, acquired a power of moving it independently of the other. 



When fquinting arifes from abfolute imperftflion in the eye, there is no cure. Where 

 it arifes from weaknefs only iivthe fight of one eye, it may in fonic inftances be got the 

 better.of by confining the perfon to the ufe of one eye by covering the other ; in this way 

 the mufcles, from conftant ufe, become perfect in the habit of directing the organ, and 

 acquire ftrength and power of adjuftment. The time required muft depend upon the 

 weaknefs of the fight, and length of time during which the mufcles have been left to 

 themfelves. 



The Author fhews, that he cornea is not, like the cuticle, devoid of life ; though, like 

 tendons and ligaments, it is neither fupplieit with red blood nor poflefles fenfibility, but is 

 made up 0,' membraneous ligaments, which arc continuations of the tunica conjunctiva and 

 ihe tendons cf the four ftrait mufcles. AVhcii wounded, it commonly unites, like other 



? living 



