156 COATS OF THE EYE. 



Dr. B. Yes, just so. The organ of vision, and the 

 mechanism by which the rays of light are transmitted 

 through it and variously modified, you must bear in 

 mind, are regulated strictly upon optical principles. The 

 whole globe ol the eye is a collection of humors, or thick 

 dense fluids arrayed in a certain order, separated from 

 one another, and enclosed by several membranes of dif- 

 ferent density and extent, called its coats. These may 

 be arranged in three divisions ; the first, serving to mod- 

 ify the rays of light ; the second, to receive the im- 

 pressions ; and the third, to transmit them to the brain. 

 And now for those of the first division. The external 

 and enveloping coat of the eye is the sclerotic, a strong 

 fibrous membrane, nearly of a spherical shape, which 

 contains and protects the contents of the eye, and pre- 

 serves its general shape. Lining this, is the choroid 

 coat, of a more delicate structure, abundantly provided 

 with blood-vessels, and furnished with a black pigment, 

 called \\iepigmentum nigrum. 



Emily. Is the white of the eye a portion of the scle- 

 rotic coat ? 



Dr. B. There indeed, you do not see the sclerotic 

 coat, because it is covered by a reflection of mucous 

 membrane, called the conjunctiva. If this were stripped 

 off, we should then see the sclerotic coat. The sclerotic 

 coat does not completely encircle the eye ; for the ante- 

 rior portion of the eye is bounded by another coat, called 

 the cornea. Its edges are intimately connected with 

 those of the sclerotic ; it is the segment of a smaller 

 sphere ; and is perfectly colourless, and transparent. 



Emily. Colorless ! why I supposed it was this col- 

 oured portion in the middle of the eye-ball. I cannot 

 conceive what else it can be, for there is no other part 

 that lean see. 



Dr. B. The part which gives the colour to the eye, 

 is a circular membrane, whose edges are connected with 

 the edges of the cornea, and which stretches across this 

 portion of the eye like a curtain. 



