ACCOMMODATION OF THE EYE. 



835 



that is in one or another meridian, and therefore for one section of the ciliary 

 muscle to contract independently, is answered in the affirmative by Dobrowolsky, 

 E. Fick, Michel and others, and in the negative by Hess. 



10. In strong accommodative effort, the lens sinks downward from gravity, 

 whatever the position of the head, on account of the relaxation of the zonula. 

 Eserin causes strong contraction of the ciliary muscle. 



The refractive action of the lens in accommodation for both distant and near 

 vision is illustrated with especial clearness by Schemer's experiment. A piece of 

 cardboard (Fig. 284, K K,) containing two small openings (S, d) separated by a dis- 

 tance less than the diameter of the pupil is held before the eye, and the observer 

 looks at two needles (p and r) placed behind each other; if the first needle (p) 

 is fixed by the observer, the second one 

 (r) appears double; and conversely. 



If the near needle (p) is fixed and the _ B 



eye is accommodated for it, the rays 

 passing from it are naturally focussed 

 at the image on the retina (p,) ; the 

 rays, however, from the distant needle 

 (r) have already come to a focus 

 within the vitreous, and, diverging, 

 form two images (r, r,,) on the 

 retina. If the right hole in the card 

 (d) be closed, the left image of the 

 distant needle (r,,) disappears. The 

 result is analogous if the eye is accom- 

 modated for the distant needle (R). 

 Then the near needle (P) forms a double 

 image (P, P,,) because the rays passing 

 from it have not yet come to a focus. 

 Closure of the right hole (d,) makes the 

 right image (P,) disappear. It must be 

 noted especially (with regard to the pro- 

 jection of the retinal image outward 

 into the field of vision) that when the 

 observing eye is accommodated for the 

 near needle, and one of the holes is 

 closed, the homonymous double image 

 of the distant needle disappears. If, 

 however, the distant needle is fixed, 

 and the opening is closed, the crossed 

 image of the near needle disappears. 



Statements have been made recently 

 that in animals and even in man the 

 sympathetic takes part in accommoda- 

 tion for distant vision. Irritation of the 

 sympathetic is said to cause the lens to become flatter. This is shown by the fact 

 that the pupillary sphincter cannot act as an auxiliary muscle of accommodation 

 when the pupil is widely dilated. 



Mammalia, birds, and reptiles exhibit the same mechanism for accommoda- 

 tion. In cephaloppds and osseous fishes, whose eyes when at rest are accom- 

 modated for near vision, active accommodation for distant vision is effected by ap- 

 proximation of the lens to the retina, in fishes by the activity of a retractor muscle 

 of the lens. In some amphibia and snakes active accommodation of the eye 

 for near vision takes place through the separation of the lens from the retina in 

 consequence of changes in the intraocular pressure. Some nocturnal animals 

 and some sensitive to light have no power of accommodation whatever. 



FIG. 284. Schemer's Experiment. 



REFRACTIVE POWER OF THE NORMAL EYE. 

 OF REFRACTION. 



ANOMALIES 



The limits of distinct vision vary greatly for different eyes. A 

 distinction is made between the far point (or resting point) and the near 

 point. The far point is the greatest distance from the eye to which an 

 object may be removed and still be seen distinctly; the near point is the 



