SENSE OF VISION. 



123 



of nerves. This seems to be a pure reflex action, in which the 

 optic nerve is the afferent^ and the third pair the efferent. The sti- 

 mulus is the presence of light. When the optic nerve is divided, the 

 Jifth pair may in some degree convey the requisite stimulus. The 

 dilatation of the pupil probably results from the elasticity of the 

 tissue of the iris after the muscular contraction has ceased. The 

 iris prevents the ingress of too much light, and also shuts off the 

 rays of light from falling on the circumference of the lens. The 

 transparent media so refract and modify the rays of light, as to 

 overcome both spherical and chromatic aberration^ and bring them 

 to a perfect focus upon the retina. 



Fig. 34.* 



35.t 



The second pair of nerves is devoted to the sense of sight, and is 

 hence called optic. The greater part of their roots may be traced 

 to the tubercula quadrige- 

 mina, which are regarded as ^^" 



the optic ganglia ; from these 

 they run forwards along the 

 base of the brain, and unite 

 in front of the tuber cinereum 

 and mammillary bodies, form- 

 ing an intimate junction call- #^:^ tr ^^^^ / 

 ed the chiasm. From this 

 point they diverge, and enter 



the orbits through the optic foramina. Part of the fibres of each 

 nerve pass to the opposite eye, part are commissural, and the re* 

 mainder pass to the eye of the same side. (Fig. 35.) 



* Longitiulinal sertion of the globe of the eye. 1. Sclerotic. 2. Cornea. 3. Choroid con- 

 nected anteriorly with (4) ciliary ligament ; and (5) ciliary processes. 6. Iris. 7. Pupil, 

 b. Retina. 9. Canal of Petit. 10. Anterior chamber, containing aqueous humour. II. Pos- 

 terior chamber. 12. Crystalline lens. 13. Vitreous humour. 14. Neurilemma of the optic 

 nerve. 16. Central artery of retina. 



t Course of fibres in the chiasm^ a. Anterior fibres, commissural between the two retime, 

 p. Posterior fibres, commissural between the thalami. a'./>'. Diagram of the preceding. 



