MAMMALIA. 



a stick. This is also shown by a toy, called the " Thauma- 

 trope," which consists of a piece of card having a small 

 wire attached to the upper part by which it may be turned 

 rapidly round by the thumb and finger. On one side of 

 the card is painted an object, such as a parrot, and on 

 the other a cage ; when the card is turned round, first the 

 bird and then the cage is presented to the eye, but the 

 image of the one has not left the eye before the other is- 

 presented to it, and the result is the uniform appearance on 

 both sides of a parrot in a cage. 



The structure of the eyes in Man and the higher animals 

 is very nearly alike, so that the same description will serve 

 for both, but those of insects and the lower orders of animated- 

 beings are very different. 



FIG. 85. SECTION* OF HUMAX EYE. 



A. Sclerotic. D. Vitreous humour. G. Crystalline lens. 



B. Choroid. E. Aqueous humour. II. Pupil. 



C. Retina. P. Cornea. I. Optic nerve. 



The eye consists of various humours of different densities 

 enclosed in membranes (fig. 85) ; the human eye is spherical 

 for the posterior four-fifths of its circumference, but the front 

 part projects a little, being formed of part of a smaller sphere. 

 The globe of the eye, all but this front part, is enclosed in a 

 firm fibrous coat called the " sclerotic," within which is a 



