THE OPTICAL SYSTEM OF THE EYE 527 



INNERVATION OF MECHANISM OF ACCOMMODATION. The ciliary 

 muscle is innervated through the 3rd nerve, its nucleus being situated near 

 the mid line under those of the pupils. Owing to the close association 

 of the nuclei on the two sides it is impossible to cause accommodation of 

 the eyes separately. From these nuclei the fibres travel down with the rest 

 of the nerve through the outer wall of the cavernous sinus, and when they 

 reach the orbital cavity are given of! to the ciliary (lenticular) ganglia, where 

 they anastomose with nerve cells the processes of which then proceed through 

 the short ciliary nerves to the eyeball. 



The position of the higher centres connected with the nuclei concerned 

 in accommodation are not definitely known ; it is believed that they come from 

 the occipital cortex. But since it is possible to carry out willed changes of 

 focus as well as subconscious ones, there must be connections with other 

 parts of the brain as well. 



THE AMPLITUDE OF ACCOMMODATION in the emmetropic (normal) eye is 

 measured by ascertaining the nearest point from the eye at which perfect vision can 

 be obtained. Since it is possible that the eyes, when examined separately, can focus 

 nearer objects than they can when used together (owing to the limitation in the power 

 of convergence), one of the eyes should be closed when making the determination. In 

 the ametropic (abnormal) eye it is necessary to determine the far point as well as the 

 near, since the former is not at infinity as it is in the emmetropic eye. But other diffi- 

 ( ulties are encountered, because in the case of hypermetropic (long-sighted) eyes an 

 object placed at infinity still requires some accommodation in order to focus it. Lastly 

 there comes the p3rsonal equation of the patient, because it is found that even when 

 apparently fully relaxed, the instillation of atropine usually causes some further relaxa- 

 tion of the accommodation ; so in the same way the instillation of eserine is usually 

 followed by a definite increase in the accommodative effort over that which can be 

 voluntarily exerted. To obtain the maximum amplitude of the accommodation these 

 drugs should therefore be used* Where a comparative and relatively inaccurate value 

 is alone required they may be omitted. Of the many methods that may be employed 

 probably the simplest is by the use of the set of trial lenses, which vary in their curvature 

 by small uniform amounts from strongly convex (plus) to strongly concave (minus). 

 The test object consists of a pin placed vertically in a board at any fixed distance from 

 the eye, a white surface being arranged behind it. A pair of spectacles to hold the trial 

 lenses are placed before the eyes, and in them are inserted two metal plates with two 

 vertical slits in them, so that each eye in turn may look at the test object through the 

 slits in the plate opposite it. To determine the near point, minus (concave) lenses of 

 gradually increasing power are placed before one of the eyes, the other being closed, 

 until one is found that just causes the image of the pin to appear double. The power 

 of the next weaker lens is therefore taken to be the correct one. To determine the far 

 point, plus (convex) lenses of increasing power are tried in a similar manner, and that 

 one taken which just does not cause appreciable doubling. The difference between the 

 power of the two lenses found in this way gives in dioptres the value of the amplitude of 

 the accommodation. Care should be specially taken to see that the value found for the 

 concave lens has the minus sign placed in front of it. Thus if it was found that the 

 far point was reached by the use of a convex lens of 2-25 dioptres, and that the near 

 point required a concave one of 7-5 D., then the amplitude is not 5-25 D. but 9-75 D. 

 Careful measurements made in the above manner show that there is for different 

 ages an average amount of accommodation. In youth the amplitude is large, but it 

 decreases uniformly to old age, and this decrease is called presbyopia (old-sight). The 

 amplitude found as a rule at different ages is approximately constant and is given in the 

 Table below. 



