408 



PRACTICAL ZOOLOGY 



spheres and cerebellum. The cerebral hemispheres are marked 

 by depressions which divide the surface into lobes or convolu- 

 tions not present in birds. In man the cerebrum constitutes 

 nine-tenths of the bulk of the brain, and the convolutions are 

 very deep. 



Sense Organs. THE EYE (Fig. 278). The eyes lie within 

 protective cavities, the orbits. In the center of each eye there 



FIG. 278. Section through human eye. 



A, choroid ; B, image on retina ; G, vitreous body ; H, cornea ; L, lens ; 

 N, retina ; p, pupil ; Pf , object ; R, iris ; Sn, optic nerve ; Stb, Stk, ciliary 

 muscle and ciliary fold ; wAu, sclerotic. (From Schnleil.) 



is an aperture for the entrance of light, forming the pupil. 

 This aperture contracts in a bright light and dilates in a faint 

 light. Directly behind the pupil is a lens-shaped body, the 

 crystalline lens. The space in front is filled with a watery fluid, 

 the aqueous humor; that behind the lens with a gelatinous 

 substance, the vitreous body. The eye is constructed on the 

 same plan as the camera of the photographer. On the retina, 

 as on the sensitive plate of the camera, there is formed an in- 

 verted and diminished image of the external world, and the 

 retina, being composed of nerve terminations sensitive to light, 



