1 24 DISEASES ,Ci-A3s I. 2. 5. i. 



ORDO II. 



Decreased Irritation. 



GENUS V. 



Decreafed A&icn of the Organs of Scnfc. 

 SPECIES. 



I. Stxhittainirritabititaf. Folly from inirritability. Dulneu 

 of perception. When the motions of the fibrous extremities 

 of the nerves of fenfe are too weak, to excite fenfation with fuf- 

 ficient quicknefs and vigour. The irritative ideas are neverthe- 

 performed, thongli perhaps in a feeble manner, as fuch peo- 

 ple do not run againtl a poft, or walk into a well. There are 

 three other kinds of folly ; that from deficient fenfation, from 

 deficient volition, and from deficient affociation, as will be men- 

 tioned in their places. In delirium, reverie,and fleep, the pow- 

 er of perception is abolifhed from other caufes. 



2. Vifus hnwinutus. Diminifhed vifion. In our approach to 

 old age our viilon becomes imperfect, not only from the. form cf 

 the cornea, which becomes lefs convex, and from' its decreafed 

 tranfparency mentioned in Clafs I. I. 3. 14 ; but alfo from the 

 decreafed irritability of the optic nerve. Thus, in the inirritative 

 or nervous fever, the pupil of the eye becomes dilated ; which in 

 this, as well as in the dropfy of the brain, is generally a fatal 

 iyinptom. A part of the cornea as well as a part of the albugi- 

 iiea in thefe fevers is frequently feen during fleep , which is ow- 

 ing to the inirfitability of the retina to light, ov to the general 

 parefis of mufcylar aQion, and in confeqence to the lefs contrac- 

 tion of the fphincter pf the eye, if it may be fo called, at that 

 time. 



In fome eyes there is an inaptitude to adapt themfelves t 

 the perception of objects at different diftarices, which I fuppofe 

 may be owing to the inirritability of thofe mufcular fibres, which 

 conftitute the ciliary procefs, fo well defcribed and explained by 

 Dr. Porter field, afcd in the Scots Medical EiTays, and fo elegany- 

 ly feen in a differed eye. It was formerly believed, and has in- 

 deed lately been again pretended, that the focus of the cryftal-- 

 1'ine humour was adapted to objects at different diftances by a 

 change of the fhape of the whole eye by the action of the exter- 

 nal mufcles, which are inferted into the tunica albuginea, and 

 j_;ive motion to it in every direction : but in an f ' is may be 



obferved| 





