914 A MANUAL OF PHYSIOLOGY 



curvature do not all lie in the same straight line ; (4) that the pupil 

 is eccentric, being situated not exactly opposite the middle of the 

 lens and cornea, but nearer the nasal side, and that in consequence 

 the optic axis, or straight line joining the centres 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, 381). (5) Muscae 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 



FIG. 392. REFRACTION IN THE (NORMAL) EMMETROPIC EYE. 



The image P' of a distant point P falls on the retina when the eye is not accommo- 

 dated. To save space, P is placed much too near the eye in Figs. 392, 393. 



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

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

 sarily defects from the freer standpoint of the physiologist, who 

 surveys 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 as abnormal conditions. 

 In the normal or emmetropic eye, parallel rays and for this 



