70 



General Biology 



Internal ear^ 

 Tympana ring 



CroS3 section 

 of jaw Jluditary n&rve, „ , „ z ; / 

 •f J A ^ e ^ uiia oblongata 



I/tnculus 



Posterior vertical 

 Semicircular canal 



Mrrupulla - - 



Branch of 

 auditory nerve 



fene-str, 

 ovale/ 



"Eustachian iub& 



^Ant&rwr vertical 

 semicircular canal 



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



layer, known as 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 which may be affected by light. The 

 chamber of the eye itself is divided in two parts by a transparent, spheri- 

 cal, crystalline lens which is 

 Craniim held in position by several 



. Fourthvm.tricle' 11 <• r^\ 



\ Cartilaginous nodufo bands ot 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 covers the entire outer 

 portion of the lens with the 

 exception of the center. 

 This central uncovered por- 

 tion is called the pupil, and 

 it is through this that 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 

 focused upon it by the lens. 



There are six muscles attached to the eyeball by means of which it can 



be moved in practically any direction (Fig. 466). 



Raprasenis cochlea, of 

 higher v&HeJoraUs "P. 



^Ampulla 



_ j Branch of 

 ^uuiiiory nerve. 



— Saccules 



Hbrizonial 

 1 sa.micircu.lar canal 



lab* 



E V .-' 



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.) 



