EYES. 89 



But horses and other animals that feed on grass, have 

 some parts of this membrane of a hght green, which 

 enables them to see with little light, and makes grass 

 an object that they can discern with greatest strength, 

 and therefore it is sometimes called tunicia uvea^ 

 from its resembling the colour of a grape. The inner- 

 most or fourth coat is called the membrana retina, 

 which is only an expansion of the optic nerve upon 

 the choroides, and encompasseth the glossy humour 

 like a net. By the continuation of the rays of light 

 upon the fine filaments of this membrane, all the 

 external images are conveyed by the optic nerves to 

 the brain. Within the coats of the eye are seated the 

 three humours that chiefly compose the eyeball ; the 

 first is the aqueous or watery humour, which lies 

 foremost and seems chiefly as a proper medium to 

 preserve the crystalline humours from injuries in case 

 of wounds, bruises, or any other external cause. 

 Behind the aqueous humour lies the crystalline lens, 

 in a very firm membrane called arena, being like a 

 spider's web — its use is to refract the rays of light 

 that pass through it, so that all the rays proceeding 

 from the same point of any object, being first refracted 

 on the cornea, may be united on the retina- — the 

 vitreous humour lies behind the crystalline, being con- 

 cave on its foreside to make a convenient lodgement for 

 the crystaUine, and its hinder part convex agreeable 

 to the globular form of the eye, upon which the tunica 

 retina and choroides are spread: this humour pos- 

 sesses a space larger than the other two, and being of 

 a hue like a light coloured green glass, is a proper 

 medium, not only to keep the crystalHne Jiumour and 

 the retina at a proper distance from each other, but by 

 its colour to prevent the rays o^ light falling too for- 



