testified that electrical stimulation of the eyeball produced accommodation, or myopic refraction, precisely as in the normal eye.
In most text-books on physiology it is stated that accommodation is controlled by the third cranial nerve, which supplies all the muscles of the eyeball except the superior oblique and the external rectus; but the fourth cranial nerve, which supplies only the superior oblique, was found in these experiments to be just as much a nerve of accommodation as the third. When either the third or the fourth nerve was stimulated with electricity near its point of origin in the brain accommodation always resulted in the normal eye. When the origin of either nerve was covered with a small wad of cotton soaked in a two per cent solution of atropine sulphate in a normal salt solution, stimulation of that nerve produced no accommodation, while stimulation of the unparalyzed nerve did produce it. When the origin of both nerves was covered with cotton soaked in atropine, accommodation could not be produced by electrical stimulation of either or both. When the cotton was removed and the nerves washed with normal salt solution, electrical stimulation of either or both produced accommodation just as before the atropine had been applied. This experiment, which was performed repeatedly for more than an hour by alternately applying and removing the atropine, not only demonstrated clearly what had not been known before, namely, that the fourth nerve is a nerve of accommodation, but also demonstrated that the superior oblique muscle which is supplied by it is an important factor in accommodation. It was further found that when the action of the oblique muscles was prevented by dividing them, the stimulation of the third nerve produced, not accommodation, but hypermetropia.
Fig. 22. Experiment Upon the Eye of a Cat Demonstrating That the Fourth Nerve, Which Supplies Only the Superior Oblique Muscle, Is Just as Much a Nerve of Accommodation As the Third, and That the Superior Oblique Muscle Which It Supplies Is a Muscle of Accommodation.No. 1. - Both nerves have been exposed near their origin in the brain, and a strip of black paper has been inserted beneath each to render it visible. The fourth nerve is the smaller one The superior oblique muscle has been advanced by a tucking operation, as this muscle is always rudimentary in cats, and unless its pull is strengthened accommodation cannot be produced in these animals. Stimulation of either or both nerves by the faradic current produced accommodation.No. 2. - When the fourth nerve was covered with cotton soaked in a normal salt solution, the application of the faradic current to the cotton produced accommodation. When the cotton was soaked in a one per cent solution of atropine sulphate in a normal salt solution, such application produced no accommodation, but stimulation of the third nerve did produce it.No. 3. When the third nerve was covered with cotton soaked in a normal salt solution, the application of the faradic current to the cotton produced accommodation. When the cotton was soaked with atropine sulphate in a normal salt solution, such application produced no accommodation, but the stimulation of the fourth nerve did produce it.No. 4. - When both nerves were covered with cotton soaked in atropine sulphate in a normal salt solution, the application of electricity to the cotton produced no accommodation. When the parts had been washed with a warm salt solution electrical stimulation of either nerve always produced accommodation. The nerves were alternately covered with the atropine-soaked cotton and then washed with the warm saline solution for an hour the electricity being applied in each condition with invariably the same result. Accommodation could never be produced by electrical stimulation when the nerves were paralyzed with the atropine, but always resulted from the stimulation of either or both when they had been washed with the salt solution. The experiment was performed with the same results on many rabbits and dogs.
Fig. 23. Pithing a Fish Preparatory to Operating Upon Its EyesThe object of this operation is to secure greater relaxation of the muscles of the eyes and head, which would work for hours, without external stimulus, if the brain cells were not destroyed by the probe.
In all the experiments all sources of error are believed to have been eliminated. They were all repeated many times and always with the same result. They seemed, therefore, to leave no room for doubt that neither the lens nor any muscle inside the eyeball has anything to do with accommodation, but that the process whereby the eye adjusts itself for vision at different distances is entirely controlled by the action of the muscles on the outside of the globe.
1. The refractive system.
2. On the Anomalies of Accommodation and Refraction of the Eye, p. 22.
3. In many animals, notably in rabbits, the internal and external recti are either absent or rudimentary, so that. practically, in such cases, there are only two recti, just as there are only two obliques. In others, as in many fish, the internal rectus is negligible.
CHAPTER V - THE TRUTH ABOUT ACCOMMODATION AS DEMONSTRATED BY A STUDY OF IMAGES REFLECTED FROM THE LENS, CORNEA, IRIS AND SCLERA
AS the conclusions to which the experiments described in the preceding chapter pointed were diametrically opposed to those reached by Helmholtz in his study of the images reflected from the front of the lens, I determined to repeat the experiments of the German investigator and find out, if possible, why his results were so different from my own. I devoted four years to this work, and was able to demonstrate that Helmholtz had erred through a defective technique, the image obtained by his method being so variable and uncertain that it lends itself to the support of almost any theory.
I worked for a year or more with the technique of Helmholtz, but was unable to obtain an image from the front of the lens which was sufficiently clear or distinct to be measured or photographed. With a naked candle as the source of light a clear and distinct image could be obtained on the cornea; on the back of the lens it was quite clear; but on the front of the lens it was very imperfect. Not only was it blurred, just as Helmholtz stated, but without any ascertainable cause it varied greatly in size and intensity. At times no reflection could be obtained at all, regardless of the angle of the light to the eye of the subject, or of the eye of the observer to that of the subject. With a diaphragm I got a clearer and more constant image, but it still was not sufficiently reliable to be measured. To Helmholtz the indistinct image of a naked flame seemed to show an appreciable change, while the images obtained by the aid of the diaphragm showed it more clearly; but I was unable, either with a diaphragm or without it, to obtain images which I considered sufficiently distinct to be reliable.
Fig. 24. Arrangements for Photographing Images Reflected From the EyeballCM, concave mirror in which the subject may observe the images reflected from various parts of her eye- C, condenser; D, diaphragm; L, 1000-watt lamp; F, forehead rest; MP, bar which the subject grasps with her teeth for the purpose of holding her head steady- P, plane mirror upon which is pasted a letter of diamond type and in which is reflected a Snellen test card twenty feet behind the subject (the mirror is just above the letter P); CAM, camera; Pr, perimeter used to measure the angle of the light to the eye; R, plane mirror reflecting light from the 1000-watt lamp upon the eye, which otherwise would be in total darkness except for the part from which the highly condensed image of the filament is reflected; B, blue glass screen used to modify the light reflected from the mirror R. When the subject read the bottom line of the Snellen test card reflected in the mirror P her eye was at rest, and when she saw the letter of diamond type distinctly it was accommodated ten diopters, as demonstrated by the retinoscope.Fig. 25. Arrangements for Holding the Head of the Subject Steady While Images Were Being PhotographedCM, concave mirror; F, forehead rest; C, condenser, MP mouthpiece; Pr, perimeter.Fig. 26. Image of Electric Filament on the Front of the LensR, rest; A, accommodation. Under the magnifying glass no change can be observed in the size of the two images. The image at the right looks larger only because it is more distinct. To support the theory of Helmholtz it ought to be the smaller. The comet's tail at the left of the two images is an accidental reflection from the cornea. The spot of light beneath is a reflection from the light used to illuminate the eye while the photographs were being taken. It took two years to get these pictures.
Men who had been teaching and
