of the hole through which the optic nerve passes. According to their position, they are known as the "superior," "inferior," "internal" and "external" recti.- The obliques are the muscles of accommodation; the recti are concerned in the production of hypermetropia and astigmatism.
Fig. 13. Demonstration Upon the Eye of a Rabbit that the Inferior Oblique Muscle is an Essential Factor in AccommodationNo. 1. - The inferior oblique muscle has been exposed and two sutures are attached to it. Electrical stimulation of the eyeball produces accommodation as demonstrated by simultaneous retinoscopy.No. 2. - The muscle has been cut. Electrical stimulation produces no accommodation.No. 3. - The muscle has been sewed together. Electrical stimulation produces normal accommodation.
In some cases one of the obliques is absent or rudimentary, but when two of these muscles were present and active, accommodation, as measured by the objective test of retinoscopy, was always produced by electrical stimulation either of the eyeball, or of the nerves of accommodation near their origin in the brain. It was also produced by any manipulation of the obliques whereby their pull was increased. This was done by a tucking operation of one or both muscles, or by an advancement of the point at which they are attached to the sclerotic. When; one or more of the recti had been cut, the effect of operations increasing the pull of the obliques was intensified.
Fig. 14. Demonstration Upon the Eye of a Carp That the Superior Oblique Muscle Is Essential to Accommodation.No. 1. - The superior oblique is lifted from the eyeball by two sutures, and the retinoscope shows no error of refraction. No. 2. - Electrical stimulation produces accommodation, as determined by the retinoscope. No. 3. - The muscle has been cut. Stimulation of the eyeball with electricity fails to produce accommodation. No. 4. - The divided muscle has been reunited by tying the sutures. Accommodation follows electrical stimulation as before.Fig. 15. Demonstration Upon the Eye of a Rabbit That the Production of Refractive Errors Is Dependent Upon the Action of the External Muscles. The String Is Fastened to the Insertion of the Superior Oblique and Rectus MusclesNo. 1. - Backward pull. Myopia is produced.No. 2. - Forward pull. Hypermetropia is produced.No. 3. - Upward pull in the plane of the iris. Mixed astigmatism is produced.Fig. 16. Demonstration Upon the Eye of a Fish That the Production of Myopic and Hypermetropic Refraction Is Dependent Upon the Action of the Extrinsic Muscles.Suture tied to the insertion of the superior rectus muscle. By means of strong traction upon the suture the eyeball is turned in its socket, and by tying the thread to a pair of fixation forceps which grasp! the lower jaw, it is maintained in this position. A high degree of mixed astigmatism is produced, as demonstrated by simultaneous retinoscopy. When the superior oblique is divided the myopic part of the astigmatism disappears, and when the inferior rectus is cut the hypermetropic part disappears, and the eye becomes normal - adjusted for distant vision - although the same amount of traction is maintained. It is evident that these muscles are essential factors in the production of myopia and hypermetropia.
After one or both of the obliques had been cut across, or after they had been paralyzed by the injection of atropine deep into the orbit, accommodation could never be produced by electrical stimulation; but after the effects of the atropine had passed away, or a divided muscle had been sewed together, accommodation followed electrical stimulation just as usual. Again when one oblique muscle was absent, as was found to be the case in a dogfish, a shark and a few perch, or rudimentary, as in all cats observed, a few fish and an occasional rabbit, accommodation could not be produced by electrical stimulation. But when the rudimentary muscle was strengthened by advancement, or the absent one was replaced by a suture which supplied the necessary countertraction, accommodation could always be produced by electrical stimulation.
After one or both of the oblique muscles had been cut, and while two or more of the recti were present and active,3 electrical stimulation of the eyeball, or of the nerves of accommodation, always produced hypermetropia, while by the manipulation of one of the recti, usually the inferior or the superior, so as to strengthen its pull, the same result could be produced. The paralyzing of the recti by atropine, or the cutting of one or more of them, prevented the production of hypermetropic refraction by electrical stimulation; but after the effects of the atropine had passed away, or after a divided muscle had been sewed together, hypermetropia was produced as usual by electrical stimulation.
It should be emphasized that in order to paralyze either the recti muscles, or the obliques, it was found necessary to inject the atropine far back behind the eyeball with a hypodermic needle. This drug is supposed to paralyze the accommodation when dropped into the eyes of human beings or animals, but in all of my experiments it was found that when used in this way it had very little effect upon the power of the eye to change its focus.
Fig. 17.No. 1. - Production of mixed astigmatism in the eye of a carp by pulling strings attached to the conjunctiva in opposite directions. Note the oval shape of the front of the eyeball.No. 2. - With the cutting of the strings the eyeball returns to its normal shape, and the refraction becomes normal.
Fig. 18. Demonstration Upon the Eyeball of a Rabbit That the Obliques Lengthen the Visual Axis in MyopiaR, rest. The eyeball is of normal length and emmetropic - that is, perfectly adjusted for distant vision. My, myopia. The pull of the oblique muscles has been strengthened by advancement and the retinoscope shows that myopia has been produced. It can easily be noted that the eyeball is longer. It was impossible to avoid some movement of the head between the taking of the two pictures as a result of the manipulation of the strings, but the rule shows that the focus of the camera was not appreciably changed by such movements.
Astigmatism was usually produced in combination with myopic or hypermetropic refraction. It was also produced by various manipulations of both the oblique and recti muscles. Mixed astigmatism, which is a combination of myopic with hypermetropic refraction, was always produced by traction on the insertion of the superior or inferior rectus in a direction parallel to the plane of the iris, so long as both obliques were present and active; but if either or both of the obliques had been cut, the myopic part of the astigmatism disappeared. Similarly after the superior or the inferior rectus had been cut the hypermetropic part of the astigmatism disappeared. Advancement of the two obliques, with advancement of the superior and inferior recti, always produced mixed astigmatism.
Fig. 19. Demonstration Upon the Eye of a Carp That the Recti Shorten the Visual Axis in HypermetropiaR, rest. The eyeball is of normal length and emmetropic. Hy, hypermetropia. The pull of the external and internal recti has been strengthened by advancement, and the retinoscope shows that hypermetropia has been produced. It may easily be noted that the eyeball is shorter. The rule shows that the focus of the camera was not appreciably changed between the taking of the two pictures.
Eyes from which the lens had been removed, or in which it had been pushed out of the axis of vision, responded to electrical stimulation precisely as did the normal eye, so long as the muscles were active; but when they had been paralyzed by the injection of atropine deep into the orbit, electrical stimulation had no effect on the refraction.
Fig. 20. Lens Pushed Out of the Axis of VisionIn this experiment on the eye of a carp the lens was pushed out of the axis of vision. Accommodation took place after this displacement just as it did before. Note the point of the knife in the pupil in front of the lens.
In one experiment the lens was removed from the right eye of a rabbit, the refraction of each eye having first been tested by retinoscopy and found to be normal. The wound was then allowed to heal. Thereafter, for a period extending from one month to two years, electrical stimulation always produced accommodation in the lensless eye precisely to the same extent as in the eye which had a lens. The same experiment with the same result was performed on a number of other rabbits, on dogs and on fish. The obvious conclusion is that the lens is not a factor in accommodation.
Fig. 21. Rabbit With Lens RemovedThe animal was exhibited at a meeting of the Ophthalmological Section of the American Medical Association, held in Atlantic City, and was examined by a number of ophthalmologists present, all of whom
