

( 2J,7 ) 



" the mecUr.Iary part of the optic nerve. Th^e convex lide of it 

 '* hnes the choroicles, the concave fide covers the furface of the vi- 

 " treous humour, terminating where the choroides folds invy'ard:5. 

 " It is an eflential organ of vifion ; on it the images of objeds are 

 •' reprefented, and their picture formed. This membrane appears to 

 " be black in infants, not fo bhick at the age of tvventy, of a gre3'iih 

 " colour about the thirtieth year, and in very old age almoft white. 

 " The retina, however, is always tnmfparent and colourlefs : any ap- 

 " parent changes therefore, of its colour, mv.il depend upon altera- 

 " tionsof the pigmentum which is feen through it. 



"Optic Nerve. 

 " Behind all the coats is fituated the optic nerve, which paffes out of 

 " the fcuil, through a fmall hole in the bottom of the orbit which 

 " contains the eye. It enters the orbit a little infle6ied, of a figure 

 " fomewhat round, but compreffed,- and is inierted into the globe 

 " of the eye, not in the middle, but a little higher, snd nearer to 

 " the nofe ; an artery runs through the optic nerve, goes ftrait 

 " through the vitreous humour, and fpreads itfelf on the membrane 

 " that covers the back fide of the cryflalline. 



" Monf. Mariotte has demonfi:rated, that our eyes are infenfible 

 *' at the place where the optic nerve enter.s : if, therefore, this nerve 

 " had been fituated in the axis of the eye itfelf, then the mid- 

 " die part of every obje6l Viould have been iuvifible, and where all 

 " things contribute to make us fee bed, we fliould not have feen at 

 " all ; but it is wifely placed by the divine artiil for this and 

 " other advantageous piirpofes, not in the middle, but, as we have 

 " already oblerved, a little higher and nearer to the r.ofe. 



" Of the Humours of the Eye. 

 *'The coats of the eye, which inveft and fupport each other, after 



Hh 



