70 



GENERAL BIOLOGY 



The darker portion of the eye acts as does the dark chamber of a 

 camera. This chamber takes up about two-thirds of the posterior part 

 of the eyeball and consists of three layers. Toward the exterior is found 

 the sclerotic ( ) coat made up of fibrous tissue and 



cartilage. Then follows a thin pigment-containing coat, known as the 

 choroid ( ) and in the inside of this a very thin 



layer, known a.s the retina ( ). It is the retina which 



is sensitive to light. Almost in the center, but a little to one side of the 

 back chamber, the optic nerve enters, spreading out on the retina, so 

 that it has a considerable area that light may affect. The chamber of 

 the eye itself is divided in two parts by a transparent spherical, crystal- 

 line lens which is held in 

 position by several bands of 

 fibers. The lens is partly 

 covered anteriorly by an 

 opaque membrane, in reality 

 a continuation of the 

 choroid, growing out of the 

 wall of the chamber on all 

 sides. This membrane is 

 known as the iris ( ), 



and it covers the entire 

 outer portion of the lens 

 with the exception of the 

 center. This central uncov- 

 ered portion is called the 

 pupil, and it is through this 

 the light enters. There are 

 pigment cells in the iris 

 which give the color to the 

 eye. 



Both of the eye-cham- 

 bers are filled with a trans- 

 parent liquid. That between 

 the cornea and the lens is 

 called the aqueous humor 

 ( ) and that 



back of the lens, which is 

 quite thick, is called the vit- 

 reous humor ( ). 

 The retina itself is quite 

 complicated, being com- 

 posed of thousands of end 

 organs of sensory nerves, 

 highly sensitive to the light 



ZV- 



Fig. 19. 



A. Diagrammatic transverse section of the head 

 of the toad showing arrangement of the parts of the 

 ear. (After Guyer from Jammes. ) 



B. The labyrinth of the right ear of the frog, 

 seen from the outer side. 



C. A diagram of the ear of the frog. col., 

 Columella ; f.o., fenestra ovalis ; Eu., Eustachian tube ; 

 lab., part of the membranous labyrinth, containing 

 endolymph ; m., mouth ; md., mandible ; peril., peri- 

 lymph ; sk., skull; tym., tympanic membrane. (B 

 and C, from Borradaile.) 



