Chap. XX] INTERNAL AND EXTERNAL SENSES 129 



exit of the optic nerve, and is the exact centre of the retina. Li 

 its centre is a tiny pit, — fovea centralis, — which is the centre 

 of direct vision ; that is, it is the part of the retina which is always 

 turned towards the object looked at. From this point the sensi- 

 tiveness of the retina grows less and less in all directions. At 

 this point (fovea centralis) are 

 found none of the fibres of the 

 optic nerve, but a great increase 

 in the number of cones, as well 

 as in their size. 



Perception of light. — 'Wlien 

 light waves fall upon the retina 

 they act as a stimulus, and it is 

 supposed that they cause chem- 

 ical changes in the rods and 

 cones which give rise to im- 

 pulses that are carried by the 

 optic nerve to the brain, and 

 result in sight. Just how this 

 is accomplished is not known, 

 but the rods contain a kind of 

 pigment which is called visual 

 purple, and this as well as the 

 pigment of the retina may func- 

 tion in these changes. 



The optic chiasm. — The fact that the two retinse and the two 

 eyeballs work in unison is largely due to the crossing of the nerve 

 fibres at the optic chiasm. The optic nerve from each eye passes 

 backward through the optic foramen, and shortly after leaving 

 the orbit the two nerves come together, and the fibres from the 

 inner portion of each nerve cross. This is called the optic chiasm, 

 and is really an incomplete crossing of fibres, as the outer fibres 

 do not cross. (See Fig. 203.) 



Aqueous humor. — The space bounded by the cornea in 

 front and by the lens, suspensory ligament, and ciliary body be- 

 hind is filled with a colorless, transparent, watery fluid, the 

 aqueous humor. This space is known as the aqueous chamber, 

 and is' partially divided by the iris into an anterior and posterior 

 chamber. 



Fig. 202. — The Posterior Half of 

 THE Retina of the Left Eye viewed 

 FROM Before. Twice its natural size, 

 s, cut edge of the sclera ; ch, choroid ; r, 

 retina ; in the interior at the middle, the 

 macula lutea with the depression of the 

 fovea centralis is represented by a slight 

 oval shade ; toward the left side the light 

 spot indicates the entrance of the optic 

 nerve or blind spot. (Collins.) 



