THE EYE. 181 



The drawing (Fig. 3) shows what is believed to be the arrange- 

 ment of parts in the retina ; very Httle, if anything, is known 

 about the action in any layer. But the whole acts much as a 

 thermopile of bismuth and antimony. The thermopile changes 

 certain waves of radiant energy, which we call heat, into electricity. 

 The retina changes certain waves of radiant energy, which we 

 call light, into electricity of some sort, which runs along the 

 nerves, by means of which the brain receives impressions. Exactly 

 in the centre of the posterior part of the retina, and at a point 

 corresponding to the axis of the eye, in which the sense of 

 vision is most perfect, is a round, elevated, yellowish spot, 

 called the Yellow spot of Sommerring, having a central depres- 

 sion at its summit, called the fovea centralis (Fig. 6). The retina, 

 in the situation of the fovea centralis, is exceedingly thin (Fig. 7), 

 so much so, that the dark colour of the choroid is distinctly seen 

 through it. It exists only in man, the quadrumana, and some 

 saurian reptiles. About one-tenth of an inch to the inner side 

 of the yellow spot in the human eye, is the entrance of the 

 optic nerve, the artery piercing its centre (Fig. 6). This is the 

 only part of the surface of the retina from which the power of 

 vision is absent. 



The aqueous humour fills the anterior and posterior chambers 

 of the eye-ball, as shown in Fig. 3. It is scarcely more than 

 water, with a trace of salt. Endothelium has only been demon- 

 strated on the inner surface of the cornea, but it probably exists 

 on the surface of both posterior and anterior chambers ; especially 

 in the foetus, where the two chambers are separate, the pupil being 

 closed by a membrane. 



The Vitreous body forms about four-fifths of the entire globe, 

 it fills the cavity of the retina, and is hollowed in front for the 

 reception of the lens and its capsule. It is perfectly transparent, 

 of the consistence of thin jelly, and consists of an albuminous fluid 

 enclosed in a delicate, transparent membrane, known as the 

 hyaloid. 



The Crystalline Lens, enclosed in its capsule (Fig. 3, /'), is 

 situated immediately behind the pupil, in front of the vitreous 

 body, and surrounded by the ciliary processes (^'), which slightly 

 overlap its margin. 



