OPTIC NERVES. 671 



CHAPTER XXII. 



VISION. 



General considerations Optic (second nerve) Genera) properties of the optic nerves Physiological anat- 

 omy of the eyeball Sclerotic coat Cornea Choroid coat Ciliary muscle Iris Pupillary membrane 

 Retina Crystalline leiis Aqueous humor Chambers of the eye Vitreous humor Summary of the 

 anatomy of the globe The eye as an optical instrument Certain laws of refraction, dispersion etc., 

 bearing upon the physiology of vision Refraction by lenses Visual purple and visual yellow and ac- 

 commodation of the eye for different degrees of illumination Formation of images in the eye Mechan- 

 ism of refraction in the eye Astigmatism Movements of the iris Direct action of light upon the iris 

 Action of the nervous system upon the iris Mechanism of the movements of the iris Accommoda- 

 tion of the eye for vision at different distances Changes in the crystalline lens in accommodation 

 Changes in the iris in accommodation Erect impressions produced by images inverted upon the retina 

 Field of indirect vision The perimeter Binocular vision Corresponding points The horopter 

 Duration of luminous impressions (after-images) Irradiation Movements of the eyeball Muscles of 

 the eyeball Centres for vision Parts for the protection of the eyeball Conjunctival mucous membrane 

 Lachrymal apparatus Composition of the tears. 



THE chief important points to be considered in the physiology of vision 

 are the following : 



1. The physiological anatomy and the general properties and uses of the 

 optic nerves. 



2. The physiological anatomy of the parts essential to correct vision. 



3. The laws of refraction, diffusion etc., bearing upon the physiology of 

 vision. 



4. The action of the different parts of the eye in the production and 

 appreciation of correct images. 



5. Binocular vision. 



6. The physiological anatomy and uses of accessory parts, as the muscles 

 which move the eyeball. 



7. The physiological anatomy and uses of the parts which protect the 

 eye, as the lachrymal glands, eyelids etc. 



OPTIC (SECOND NERVE). 



The bands which pass from the tubercula quadrigemina to the eyes are 

 divided into the optic tracts, which extend from the tubercula on either side 

 to the commissure, or chiasm, the chiasm, or the decussating portion, and 

 the optic nerves, which pass from the chiasm to the eyes. 



The optic tracts arise each one by two roots, internal and external. The 

 internal roots, which are the smaller, arise from the anterior tubercula quadri- 

 gemina, and pass through the internal corpora geniculata, to the optic chiasm. 

 The external roots, which are the larger, arise from the posterior part of the 

 optic thalami, pass to the external corpora geniculata, from which they receive 

 fibres, and thence to the chiasm. 



Partly by anatomical researches (Wernicke) and partly by experiments on 

 the cerebral cortex in the lower animals and pathological observations on the 

 human subject, it has been shown that fibres from the apparent origin of the 

 optic tracts pass backward to the gray matter of the occipital lobes of the 

 cerebrum. It has also been stated by Stilling that fibres pass to the medulla 



