424 ORGANS OF TEE SENSES. 



which (with the exception of the nervous portions) are formed 

 in a great measure in the embryo, by deep and extremely 

 complicated growths of the external integument. To the 



Fig. 111. The globe of the eye (vertical section).* 



eyeball or globe of the eye thus constituted, accessory organs 

 are attached, intended either to move it (muscles of the eye), 

 or to protect it from external injury (eyelids and lachrymal 

 system). 



We shall successively consider: 



1. Physical dioptric apparatus. 



2. Accessory membranes which maintain and modify its 

 function. 



3. Sensitive membrane or retina. 



4. Appendages of the eye. 



1. The dioptrical apparatus. 



A. Media of the Eye. The dioptrical apparatus consists 

 of all the transparent media through which the rays of light 

 pass in order to reach the sensitive membrane situated at the 

 bottom, or fundus, of the eye ; these are, beginning in front, 

 the cornea, the aqueous humor, the crystalline lens, and the 

 vitreous humor / the cornea, which, in an anatomical point 

 of view, forms a part of the covering of the eye, in a physio- 

 logical point of view constitutes one of these media. 



* 1, Sclerotic. 2, Choroid. 3, Retina. 4, Crystalline lens. 5, Hyaloid 

 membrane. 6, Cornea. 7, Iris. 8, Vitreous body. (J. C. Dalton, "Human 

 Physiology.") 



