ORGAN OP VISION. 229 



chiasm, the root of the opposite side. MM. Rolando and Flourens, 1 

 too, found in their experiments, that when one cerebral hemisphere was 

 removed, the sight of the opposite eye was lost. We may conclude, 

 then, in the present state of our knowledge, that there is not simply a 

 junction, or what the French call adossement, of the optic nerves ; but 

 that they decussate at the sella turcica. 2 



The eye proper receives numerous vessels, ciliary arteries and veins 

 and several nervous ramifications, ciliary nerves the greater part 

 of which proceed from the ophthalmic ganglion of the fifth pair. The 

 following are the dimensions, &c., of the organ, on the authority of 

 Petit, Young, Gordon, and Brewster. 



Eng. inch. 

 Length of the antero -posterior diameter of the eye 0'9 1 



Vertical chord of the cornea - 



Versed sine of the cornea 



Horizontal chord of the cornea 



Size of pupil seen through the cornea 



Size of pupil diminished by magnifying power of cornea 



Radius of the anterior surface of the crystalline 



Radius of posterior surface 



0-45 

 0-11 

 0-47 



- 027 to 0-13 



- 0-25 to 0-12 



0-30 

 022 



Principal focal distance of lens - 1*73 



Distance of the centre of the optic nerve from the foramen centrale of * 



Sommering ........ ... 0'11 



Distance of the iris from the cornea 0-10 



Distance of the iris from the anterior surface of the crystalline - - 002 



Field of vision above a horizontal line 

 Field of vision below a horizontal line 

 Field of vision in a horizontal plane 

 Diameter of the crystalline in a woman 

 Diameter of the cornea 

 Thickness of the crystalline - - ^ 

 Thickness of the cornea 



50) 



70 I L *"~ 

 150' 



above fifty years of age - - 0'378 



0-400 

 0-172 

 0-0424 



It is proper to remark, that all these measurements were necessarily 

 taken on the dead organ, in which the parts are by no means in the 

 same relative situation as when alive ; and this is a cause why many 

 of the phenomena of vision can never be determined with mathematical 

 accuracy. 



3. ACCESSORY ORGANS. 



The visual organs being of an extremely delicate texture, it was of 

 obvious importance, that they should be guarded against deranging 

 influences. They are accordingly provided with numerous parts that 

 afford them protection, and enable them to execute the functions for 

 which they are destined. They are, in the first place, securely lodged 

 in the bony cavities called orbits, which are of a conical figure, with 

 the apices directed inwards. In the truncated apex the foramen opti- 

 cum is situate, by which the optic nerve enters the orbit. Here are, 



' Recherches Experimental sur le Systeme Nerveux, 2de e"dit., Paris, 1842. 



* See, on this subject, Adelon, Physiologic de 1'Homme, i. 402, 2de edit., Paris, 1829, and 

 Bostock's Physiology, edit, cit., p. 709. 



a According to Young, Philos. Transact., P. i. p. 46, Lond., 1801, the field of vision inter- 

 nally is 60, externally 90; according to Purkinje, (Rust's Magazine, B. xx. Berlin, 1825,) 

 internally 60, externally 100. 



< For the dimensions of different parts of the eye see Krause, in Meckel's Archiv ftir 

 Anatomie und Physiologic "fur 1832; and Longet, Traite de Physiologie, ii. 41, Paris, 1850. 



