Cl*ss I. 2. 5. 3. OF IRRITATION. t&£ 



obferved, that if tfce common aetbns of the mufefes arretted the 

 focus of the eye, every motion of the eye-ball, when we attend 

 to objects at any diftance, muft difturb our vifion. At the fan 

 time though it is pomble, that a violent action of all tht mufcies 

 together, io as to counter-balance each other, and keep the eye 

 motkmlefs as when we look gainfully at a very diilant object, 

 may in feme degree afreet the form oi it ; yet that the contrac- 

 tion of the ciliary procefs perpetually adapts rhe focus of the cry i- 

 talliue lens to the diftance of the retina is not to be difputed. 



There have been inftanoes of fome, who could not diitinguifh 

 certain colours ; and yet whefe eyes, in other refpects, were not 

 imperfect. Philof. Tranfact. Which feems to have been 

 owing to the want of irritability, or the inaptitude to action, of 

 •fome dalles of fibres which compofe the retina. Other perma- 

 nent defects depend on the difeafed fcate of the external organ. 

 Oafs I. 1. 3. 14. 1. 2. 2. 13. IV. 2. t. 11. 



3. Muf:£ vclitc.rJes. Dark fpors appearing before the eyes, 

 and changing their apparent place with the motions of the eyes, 

 are owing to a temporary defect of irritability of thofe parts of 

 the retina, which have been lately expofed to more luminous ob- 

 jects than the other parts of it, as explained in Sect. XL. 2. 

 Hence dark fpots are ittn on the bed-clothes by patients, when 

 the optic nerve is become le/s irritable, as in fevers with great 

 debility \ and the patients are perpetually trying to pick them off 

 with their fingers to difcover what they are ; for thefe parts of 

 the retina of weak people are fooner exhauded by the ftimulus 

 of bright colours, and are longer in regaining their irritability. 



Other kinds of ocular fpectra, as the coloured ones, are alfo 

 more liable to remain in the eyes of people debilitated by fevers, 

 and to produce various hallucinations of fight. For after the 

 contraction of a mufcle, the fibres of it continue in the lad fitu- 

 aiion, till fome antagonist mufcies are exerted to retract them ; 

 whence, when any one is much exhausted by exercife, or by want 

 of fleep, or in fevers, it is ealier to let the fibres of the retina re- 

 main in their felt fituation, after having been ftimulated into 

 contraction, than to exert any antagonift fibres to replace them. 



As the optic nerves at their entrance into the eves are each of 

 them as thick as a crow-quill, it appears that a great quantity of 

 fenforial power is expended during the day in the perpetual ac- 

 tivity of our feme of vifiori, befides that ufed in the motions of 

 the eye-balls and eye-lids ; as much I luppofe as is expended in 

 the motions of our arms, which are fupplied with nerves of 

 about the fame diameters. From hence we may conclude, that 

 the light fhouid be kept from patients in fevers vith debility, to 

 prevent the unneceflary exhauftiOn of the fenforial power. 



And 



