3S6 



ZOOLOGY 



catch the waves. The tube {external auditory meatus) and the 

 fuiinel or pinna constitute the external ear. 



374. The Eye. — The eyes of vertebrates are a single pair of 

 organs lying imbedded in an orbit of cartilage or bone, within 

 which they have considerable freedom of motion. Six muscles, 



Pig. 175. 



Fig. 175. Diagrammatic horizontal section through the right eye of Man. The line a ^ is the 

 axis of vision. The optic nerve leaves the eye on the median side of this line, a.c, central artery ; 

 a./i., aqueous humor; &., blind spot, the entrance of the optic nerve; c, conjunctiva; c/i., choroid 

 layer of the eyeball; c.l.. crystalline lens; c.fn.c, circular fibres of the ciliary muscles; c.m.r., radial 

 fibres of the ciliary muscles; c'o., cornea, the transparent portion of the sclerotic; c.^., ciliary process; 

 €.s., canal of Schlemm, a lymphatic vessel; /o., fovea centralis, the point of clearest vision; o.w., 

 optic nerve; o.s,, ora serrata, the anterior wavy margin of the visual portion of the retina; r, the 

 retinal layer; sc, sclerotic layer; sh.^ sheath of the optic nerve; v.A., vitreous humor. 



Questions on the figure. — Which is the essential sensory portion of the eye? 

 Which parts are concerned in bringing the rays of light to a focus? How many 

 refractive surfaces are present? How many refractive media? Which portions 

 of the eye are primarily protective and supportive? What is the function of the 

 various parts of the choroid layer? In what way is an image formed on the retina, 

 of objects in front of the lens? 



four straight {rectus) and two oblique, serve to move the eyeball. 

 These muscles receive the third, fourth, and sixth of the cranial 

 nerves. In the higher forms muscular folds of the skin serve 

 to protect the eye in front. The upper and lower lids act 

 vertically, but the third {nictitating membrane) works from the 



