392 F-SJ of different Anwmls. fBook IX. 



prey in the night, or in dark places (and confequently 

 rrmft have their eyes fo formed as that a few rays of 

 light may make a lively impreffion on the retina), have 

 their pupils in the day-time contracted into a very nar- 

 row fpace, as a great number of rays would opprcb 

 their nice organs ; while in the night, or where the 

 light is faint, they open the pupil, and very fully adir.it 

 the rays. In the fame way, when the retina is in- 

 flamed, a great number of rays of light would occafion 

 a painful fenfation ; therefore the pupil is contracted : 

 on the contrary, in dying people, or in a beginning 

 amaurofiSj it is generally dilated, as the eyes on fuch 

 cccafions are very difficultly affected, and in fome 

 meafure infenfible. See Plate XV. Fig. 3,4, 5. 



e The pofterior part of the choroid coat, which is- 

 called tapetum, is of different colours in different 

 creatures. For oxen, feeding moftly on grafs, have 

 this membrane of a green colour, that it may reflect 

 upon the retina all the rays of light which come from 

 the objects of that colour, while other ray.\ are ab- 

 forbed : thus the animal fees its food better than it 

 does other objects. Cats and owls have their tapetum 

 of a whitiih colour ; and for the fame reafons have the 

 pupil veiy dilatable, and their organs of vifion acute: 

 and we mail find, that all animals fee more or lefs dif- 

 tinctly in the dark, according as their tapetum ap- 

 proaches nearer to white or black colour. Thus dogs, 

 who have it of a greyim colour, diftinguifh objects 

 better in the night than man, whofe tapetum is dark 

 brown, and who, I believe, fees worfl in the dark of 

 any creature; it being originally defigned that he mould 

 reft from all kinds of employment in the night time. 

 The difference then of the colour of the tapetum, as 

 indeed .the fabric of any other part in different creatures, 

 always depends on Ibme particular advantage accruing 



to 



