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Legend of the preceding figures

Because it was impossible to depict the eye muscles in the large tables of muscles, and the eye would have to be removed from the skull to show the muscles, we have placed a picture of them here. In the first, one side of the eye is shown together with its muscles still in place. The second shows the first six muscles of the eye laid out on the ground from their insertion with the seventh kept on the optic nerve. The character legend will be set out on the next page.

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A1, 2 Anterior surface of the eye, in which the iris is situated.

B1, 2 Optic nerve,1 resected where it first comes out of the skull2 into the region of the eyes.

C, D, E1 Three muscles occur on this side3, no muscle having yet been freed from its position.

F, G1 Two of the intervals between the first six muscles, filled with fat, appear on this side.

H, I, K, L, M, N The first six muscles of the eye: H marks the muscle by which the eye is moved upward; I the one by which it is moved downward, M and N the ones by which it is rotated4, K by which it is moved to the greater angle of the eye, L by which it is taken to the lesser angle.

O The seventh muscle 5 of the eye, freed nowhere from the optic nerve and completely stripped of the fat that otherwise covers it.

Eyes must be moved by voluntary motion6

Since the vision of the eyes is not accomplished from the sides nor from the posterior, superior, or inferior areas, or in any way other than along a straight line from the pupil7, we should see very little indeed if the eyes were completely immobile8. For this reason the Maker of our body created them in such a way that they could be rotated a great deal, and besides this he made the neck easy to move and the two eyes with a distinct space between them. If it was required that they be moved with a voluntary motion, it was necessary that they be granted muscles as well.

The motions of the eyes9

But since there are three motions of the eyes, one by which they are moved inward [adduction] toward the nose and outward [abduction] toward the small angle, the second when they are turned upward [elevation] toward the brows and downward [depression] toward the cheeks, and the third by which they are rotated in a rather obscure motion, you will justly infer that muscles needed to be made in the same number as there are motions, and that it was necessary that two muscles be provided for each of the motions that can be divided into two.

The first six muscles10

Of these, it was fitting that two (K and L in fig. 2) be placed on the sides, one by each angle of the eye, two others (I and H in fig. 2) below and above respectively, and the fifth and sixth (M and N in fig. 2) to be in charge of rotational movement. Of these six muscles, the first four, as they are in charge of straight motions of the eye, so they have a straight location and are all like each other everywhere. Their heads take their origin as if from the hard cerebral membrane11 (see α and γ in fig. 17, ch. 14, Bk. 7) covering the optic nerve and from a nerve of the second cerebral pair 12 (K in figs. 1, 2, ch. 2, Bk. 4) immediately after the exit of the optic nerve from the skull. Their belly is wider than it is deep, but it appears almost rounded, and it rests upon the posterior region of the eye and the optic nerve along the length of its course from the skull to the root of the eye, and when the belly reaches the region of the eye halfway between the front and the back (where it is widest), it finishes in a wide and membranous change into a tendon.13 This ends beyond the middle area of the eye, and is inserted next to the iris 14 (x in the figures below) or great circle of the eye visible in its anterior part and separating the black (so to speak), in a straight line (but following the width of its body) in the hard tunic (figs. 16 and 17, ch. 14, Bk. 7) of the eye as if in a bone that must be moved. The muscles making the rotational motions of the eye resemble the previous ones in form and origin, but they are smaller and differ from them in their course and insertion. The first [N] takes its origin 15 out of the region of the lesser angle 16 from the hard membrane of the cerebrum surrounding the optic nerve and is inserted in the lower surface of the eye via its own thin, sinewy end at the curving line next to the iris, like the other four. The second [M] arises 17 out of the region of the greater angle 18 from the membrane of the optic nerve and inserts its thin, sinewy end in the hard tunic of the eye at the somewhat curving line at the upper part of the eye. From this it happens that the six muscles of the eye vary only in location and in their insertion in the hard tunic of the eye, and this variation is slight. The first four, as I was just saying, are quite straight and are inserted in a straight line; the other two are more curved, making their insertion in an curving line. Although they induce rotation, they also move the eye upward and downward. The course of all the thin, sinewy ends is situated halfway between the hard tunic of the eye and and the adherent tunic (η , η in fig 18, ch. 14, Bk. 7). Thus all the muscles together (all of figure 1), so long as they keep their own place, form a shape like a top whose cone is their origin and the base their insertion.

Fat between the muscles of the eyes

The eye muscles are connected to the optic nerve over an extremely short course, since in the space that they collectively form a large amount of slightly hard, very white fat 19 (F, G in fig. 1) is contained, and this in turn is wrapped around another muscle.

The seventh muscle of the eye 20

Besides the six eye muscles already mentioned, there is another large muscle (O in fig. 2), surrounded on all sides by those muscles and by the fat of which we have spoken. By itself it takes a shape like the one which the six prior muscles together made. This muscle also takes its beginning from the hard membrane wrapping the optic nerve, doing so a little anteriorly to the six preceding muscles, for their origin is more removed from the body of the eye than that of

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the muscle we are now describing, whose origin is similarly fleshy as is all the rest of the muscle's body. It surrounds the optic nerve in a ring and extends forward from its beginning, widening in a conical shape until it makes contact with the posterior part of the eye, and is inserted there in a circular manner. The muscle makes this insertion not near the optic nerve but approximately where the eye begins to be widest. In the same way that I said fat is deposited between this muscle and the six previous, so also fat is collected between the nerve and this muscle in the interval by which it is separated from the nerve. This muscle has no inscription at all except perhaps one on its lower side extending lengthwise. I therefore wonder why this was not counted by other anatomists as one muscle, but by some as two, by others as three, though all have described it in no casual way.

The function of the seventh muscle

Accordingly I also fear that they have not described the use of this muscle very accurately when they say that its sole function is to keep the eye in its place and ascribe it no additional function. But the six previous muscles could do this job better, as their outer surface is attached by many fibers to the membrane surrounding the eye socket carved in the skull (the first figure shows these things). 21 Indeed, these first six come closer to the bone of the head where the nerve comes out of the skull; for at just the spot where the nerve emerges they take their origin from its hard membrane , and they hide the seventh on every side. But perhaps there will be those who will argue that the origin of these muscles comes not from the hard membrane of the brain but from the membrane that covers the bone, although the seventh muscle is everywhere many intervals distant from the membrane covering the bone, which would certainly not have been the case if it had been needed to retain the eye in its place or in any way to draw it toward the brain. If perhaps you have scrutinized its nature carefully, you will grant that it covers the optic nerve like an integument, and after that you will agree that it moves the eye in straight motions according as it tightens or relaxes these or those fibers of its body.

Appendix

The First Six Muscles of the Eye (1555 version)

The first four muscles moving the eye

Two muscles are rightly adapted to each of the motions; the first of these (K in fig. 2) stands at the inner side of the eye and moves the eye inward. The second (L in fig. 2), placed at the outside, draws it outward. The third (H in fig. 2) is above, and moves the eye upward as the fourth (I in fig. 2), which is located below, moves it downward. The four muscles, as they are responsible for straight movements of the eye, have a correspondingly direct position and are everywhere alike. Their heads take their origin from the hard membrane of the brain (see α and γ in fig. 17, ch. 14, Bk. 7) that covers the optic nerve, and from the nerve of the second pair (K in figs. 1, 2, ch. 2, Bk. 4) of cerebral nerves soon after the optic nerve drops out of the brain cavity; their belly is wider than it is deep but nearly round, and when this belly reaches the region of the eye that is halfway between the front and the back it ends in a wide, membranous transition to tendon that runs forward beyond the middle of the eye and is inserted in the hard tunic of the eye (fig. 16 and 17, ch. 14, Bk. 7) as if into a bone to be moved, not far from the iris (x in the same figures) and the greater circle of the eye on a straight line along the breadth of the eye.

Fifth and sixth muscles

The fifth (M in fig 2) and sixth (N in fig. 2) muscles are the agents of rotation and correspond in form and origin to the four just mentioned. But they are smaller, and differ from them in their course and place of insertion. One takes its origin out of the hard cerebral membrane surrounding the optic nerve, from the area of the lesser angle; its thin, sinewy end is implanted diagonally in the lower part of the eye near the iris. The other arises from the hard cerebral membrane in the region of the greater angle and inserts its thin, sinewy end on the upper side of the eye, also on a diagonal line. Though both of these serve chiefly for rotation, they also move the eye somewhat upward and downward. As these six muscles all together preserve their own position and are connected together, they make a shape like a top (first figure as a whole) whose cone is their origin and whose basis is their insertion.