796 A MANUAL OF PHYSIOLOGY 



the optic axis, or straight line joining the centre of curvature of the 

 lens and cornea, does not coincide with the visual axis, or straight 

 line joining the fovea centralis with the centre of the pupil, which is 

 also the straight line joining the centre of the pupil and any point to 

 which the eye is directed in vision. The angle between the optic 

 and visual axis is about 5 (Fig. 286). (5) Muscat volitantes, the 

 curious bead-like or fibrillar forms that so often flit in the visual field 

 when one is looking through a microscope, are the token that the 

 refractive media of the eye are not perfectly transparent at all parts ; 

 they seem to be due to floating opacities in the vitreous humour, 

 probably the remains of the embryonic cells from which the vitreous 

 body was developed. (6) Lastly, it may be mentioned that slight 

 irregularities in the curvature of the lens exist in all eyes, so that a 

 point of light, like a star or a distant street-lamp, is not seen as a 

 point, but as a point surrounded by rays (irregular astigmatism). In 

 bringing this review of the imperfections of the dioptric media of the 

 normal eye to a close, it may be well to explain that what are defects 



FIG. 297. REFRACTION IN THE (NORMAL) EMMETROPIC EVE. 



The image P of a distant point P falls on the retina when the eye is not accommodated* 

 To save space, P is placed much too near the eye in Figs. 297 299. 



from the point of view of the student of pure optics are not 

 necessarily defects from the freer standpoint of the physiologist, who 

 suneys the mechanism of vision as a whole, the relations of its 

 various parts to one another and to the needs of the organism it -has 

 to serve, the long series of developmental changes through which it 

 has come to be what it is, and the possibilities, so far as we can limit 

 them, that were open to evolution in the making of an eye. The 

 optician may perhaps assert, and with justice, that he could easily 

 have made a better lens than Nature has furnished, but the physio- 

 logist will not readily admit that he could have made as good an eye. 



While the defects hitherto mentioned are shared in 

 greater or less degree by every normal eye, there are certain 

 other defects which either occur in such a comparatively 

 small number of eyes, or lead to such grave disturbances 

 of vision when they do occur, that they must be reckoned 



