PHYSIOLOGICAL ANATOMY OF THE OPTIC NERVES. 767 



fibres. These descriptions are from preparations made with chloride of gold ; but the 

 plates by which they are illustrated are somewhat unsatisfactory. 



According to the views of those who have described the so-called taste-beakers, sapid 

 solutions find their way into the interior of these structures through the taste-pores and 

 come in contact with what have been called the taste-cells, these structures being directly 

 connected with the terminal filaments of the gustatory nerves. 



CHAPTER XXIV. 



VISION. 



General considerations Physiological anatomy and general properties of the optic nerves Physiological anatomy of 

 the eyeball Sclerotic coat Cornea Membrane of Descemet, or of Demours Ligamentum iridis pectinatum 

 Choroid coat Ciliary processes Ciliary muscle Iris Pupillary membrane Retina Crystalline lens Aqueous 

 humor Chambers of the eye Vitreous humor Summary of the anatomy of the globe The eye as an optical 

 instrument Laws of refraction, dispersion, etc., bearing upon the physiology of vision Theories of light Re- 

 fraction by lenses Myopia and hypermetropia Formation of images in the eye Mechanism 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 Accommodation of the eye to vision at different distances 

 Changes in the crystalline lens in accommodation Action of the ciliary muscle Changes in the iris in accom- 

 modationErect impressions produced by images inverted upon the retina Single vision with both eyes Cor- 

 responding points The horopter Appreciation of distance and of the form of objects Mechanism of the stereo- 

 scope Duration of luminous impressions Irradiation Movements of the eyeball Muscles of the eyeball Parts 

 for the protection of the eyeball Eyelids Muscles which open and close the eyelids Conjunctival mucous 

 membrane Lachrymal apparatus Composition of the tears. 



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

 lowing : 



1. The physiological anatomy and the general properties 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 the functions of accessory parts, as the muscles 

 which move the eyeball. 



7. The physiological anatomy and the functions of the parts which protect the eye, as 

 the lachrymal glands, eyelids, etc. 



Physiological Anatomy of the Optic Nerves. The optic nerves, or optic tracts, take 

 their origin, each by two principal roots of white matter and a few filaments from what is 

 described as the gray root, chiefly from the tubercula quadrigemina, but in part from 

 those portions of the encephalon over which the nerves pass to go to the eyes. The 

 internal white root arises from the posterior, and the external white root, which is the 

 larger, from the anterior tuberculum. The gray root is situated in front of and above 

 the optic commissure and is a dependence of the gray matter which eovers the internal 

 surface of the optic thalamus. It arises from the gray matter which constitute- tin.- ante- 

 rior floor of the third ventricle, in the form of delicate filaments which join the optic 

 nerves at this point. 



The apparent origin of the optic nerves is from the tubercula quadripeniina, receiving 

 filaments from the corpora geniculata, the optic thalami, the peduncles of the cerebrum, 

 the anterior substantia perforata, the tuber cinereum, and the lamina terminalis. It has 

 thus far been found impossible to trace all these roots to their true origin in the cerebral 

 substance ; but experiments upon the lower animals, in which it has been shown that 



