390 Peculiarities in the Eyes [Book IX, 



The fore part of the eye is filled by a fluid tranf- 

 parent like the others, but as thin as water, and it is 

 therefore called aqueous ; this occupies all the fpace 

 between the cryftalline lens and the prominent cornea. 

 The iris floats loofely in this fluid, and divides it into 

 two parts called chambers, which communicate with 

 each other through the pupil. The pofterior chamber 

 is that fpace contained between the pofterior furfacc 

 of the iris and the lens ; the anterior is that between 

 the anterior part of the iris and the cornea. 



The eye receives its blood from the internal carotid 

 artery. The optic nerve does not enter it immedi- 

 ately behind the pupil at its pofterior part, but rather 

 towards the nofe, fo that the diftance between the pupil 

 and optic nerve is greater when meafured round the 

 external fide of the eye next the forehead, than when 

 the internal furface is meafured next the nofe. At that 

 part of the eye where the optic nerve enters, no fenfe 

 of vifion can be excited. 



The mufcles of the eye have been already defcribed 

 jn another part of the work. For the human eye, 

 fee Plate XV. Fig. i, and 2. 



The father of the prefent Dr. Monro, of Edinburgh, 

 has publifhed, in his comparative anatomy, fome excel- 

 lent remarks on the variety in the eyes of different ani- 

 mals, thnn which no more ftriking inftance can be pro- 

 duced of the wifdom and defign which pervades creation. 



f All quadrupeds have, he obferves, at the internal 

 canthus of the eye, a flrong firm membrane with a 



accounted for from the different refractive power of the medium 

 in which they live. The rays of light, in pafling out of one me- 

 dium into another, undergo a refraction proportioned to the dif- 

 ference^of their denfities. As water, therefore, is a more denfe 

 medium than air, the eyes of fuch animals as inhabit the former 

 muft have a greater refraftive power than thofe which live in the 

 latter, for the produ&ion of diftincl vifion, 



car- 



