DIOPTRIC MECHANISMS OF THE EYEBALL 547 



distance is about ten inches. A smaller distance than this is rarely made 

 use of even in youth, when the point of near vision is only three or four 

 inches from the eye, on account of the effort required to converge the axes of 

 the two eyes to this extent. The effect of the gradual diminution in the elas- 

 ticity of the lens is noticed as soon as the point of near vision recedes beyond 

 ten or twelve inches, i.e. 25 to 30 cm. This occurs as a rule between forty- 

 five and fifty, when we begin to experience difficulty in reading small print, 

 since the visual angle subtended by such print at a distance over ten or 

 twelve inches is too small to allow of distinct vision. The condition of 

 presbyopia, is remedied by employing reading glasses, i.e. by wearing 

 spectacles which converge the rays of light and so enable small objects 

 to be brought nearer to the eye than its near point. It is evident that these 

 glasses must be strengthened continually with advancing age. No trouble 

 is experienced in seeing distant objects, the refraction of the eye at rest 

 remains as before. The dimness of vision in extreme old age for distant 

 as well as near objects is due to changes, not in the refractive power of the 

 eye, but in the transparency of the refractive media, such as cataract, i.e. 

 opacity of the lens, opacity of the cornea, and so on. The former condition 

 can often be remedied by extracting the lens. Strong convex glasses (ten 

 dioptres) must then be used to take the place of the lens. 



THE COMPARATIVE PHYSIOLOGY OF ACCOMMODATION 



The mechan'sm of accommodation which we have studied in man is found with very 

 little modification throughout the whole group of mammalia, though, in the domestic 

 animals at any rate, the range of accommodation is very much less than in man. On 

 examining other types of animals we meet, as was shown by Beer, an amazing variety 

 of methods by which the range of the eye may be altered. In order to bring distinct 

 images of objects at various distances on to the retina, practically every possible focusing 

 method is made use of in one type or another of the animal kingdom. The following 

 details are taken from Beer's papers. 



In birds the eye, like that of man, is normally focused for distant objects, and accom- 

 modation for near objects is accomplished by a change in curvature of the anterior 

 surface of the lens. Whereas, however, in man the suspensory ligament is relaxed 

 by a drawing forwards of the choroid membrane, in the bird's eye this relaxation is 

 effected by a drawing backwards of the posterior lamina of the cornea, where it breaks 

 up into the ligamentum pectinatum iridis. In these eyes the main attachment of the 

 suspensory ligament is to the ligamentum pectinatum ; the retraction of this ligament 

 is effected by a special muscle known as Grampian's muscle, which corresponds to the 

 ciliary muscle in man, but unlike this consists of striated voluntary muscle. This 

 movement of the posterior elastic lamina of the cornea can be easily shown by passing 

 two needles through the corneo -sclerotic junction until their points lie in the anterior 

 chamber. On exciting Crampton's muscle electrically, the outer end of the needle 

 moves forwards, showing that the deeper part of the corneo -sclerotic junction is being 

 pulled backwards towards the ciliary portion of the eye (Fig. 275). The histological 

 character of the muscle of accommodation in birds seems to be connected with the 

 rapid accommodation that is necessary when a bird swoops down towards the ground 

 to pick up some food insect. Moreover, since binocular vision is not present in many 

 birds, and convergence of the optic axes must be minimal, it is probable that the con- 

 tractions of Crampton's muscle play a great part in guiding the movements of the 

 bird, and especially in aiding it to judge distances. In our selves such judgment is 

 very faulty without the co-operation of the two eyes. 



