THE TUNICS OF THE EYEBALL. 555 



nerve, the sensation produced is simply light, and this is thought 

 of as if it came through the eye from the outer world. 

 The study of sight may then be divided into : 



1. The path the light takes on its way through the eye to reach 

 the retina. 



2. The molecular changes in the retina which give rise to 

 stimulation of the optic nerves. 



3. The sensations arising in the sensorium as the result of the 

 molecular changes set up in the cerebral nerve cells by the im- 

 pulses from the optic nerve. 



4. The visual perceptions and judgments which our conscious- 

 ness is capable of elaborating from the visual sensations. 



THE TUNICS OF THE EYEBALL. 



The organ of vision of vertebrate animals is inclosed in a firm 

 case of fibrous tissue called the sclerotic coat, which is continuous 

 with the sheath of the optic nerve, and is seen between the eyelids 

 under the transparent conjunctiva, and is commonly known as 

 the white of the eye. It gives shape and protection to the eye, 

 and though translucent, is not transparent. In front a round 

 window-like portion, called the cornea, forms the most anterior 

 segment of this protecting covering of the eyeball. The cornea is 

 distinguished from the sclerotic not only by its glass-like trans- 

 parency, but also by being part of a lesser sphere than the scle- 

 rotic, and thus it projects a little more than the rest of the bulb. 



Closely attached to the inside surface of the sclerotic is a soft, 

 thin, black vascular sheet of tissue which supplies the eyeball 

 with blood, being made up chiefly of bloodvessels and stellate, 

 pigmented, connective-tissue cells. Its outer layer is traversed 

 by arteries and veins of relatively large size, and its inner layer 

 is practically composed of a dense network of close-meshed capil- 

 lary vessels. As the cornea is approached, the choroid is pecu- 

 liarly modified and thrown into folds, called ciliary processes, 

 forming a series of vascular projections, which radiate from the 

 margin of the cornea. At the edge of the cornea the choroid is 

 more firmly attached to the sclerotic by a circular muscle (the 



