My dear and respected Sir,

Sir Ch. Bell’s statement, that the eyeballs are turned upwards and inwards, when consciousness begins to fail, is quite correct; I saw it with remarkable distinctness in a young hysterical person, who was reputed to take scarcely any food, the eyelids being so lean and thin, that even the colour of the iris could be perceived. But his explication is, as you supposed, quite a mistake.¹ Bell was not aware of the function of the muscles of the eye. I pointed out (1847), that the oblique are in the same relation to the innervation by the will as the straight, and that a combined action is required in every movement upwards (rectus superior et obliq. inferior) and downwards (rectus inferior et obliquus superior).² The leading principle of my researches was this; that, in every mechanism, we always ought to determine in the first place the mouvements, and, after these beeing wel determined, in the second place, the muscles, by which they are produced. You may find an abstract of my doctrine (generally adopted) in Soelberg Wells, Treatise on the diseases of the eye. London 1869, p. 548 seqq., which book Mr. Bowman will be happy to send you, at your desire.³

Now, I think, it is evident, that the mouvement of the eyeballs upwards and inwards, when consciousness begins to fail, is not at all a relaxation of the muscles, but is to be considered as the result of contraction of the r. superior, r. internus and (if the vertical meridian acquires the same inclination as in the normal sight) of the obliquus inferior: Other groups of muscles are also in contraction during the sleep, especially the sphincteres.⁴

The conditions of failing consciousness by sleep and of being lost in meditation are quite different and correspond to different conditions of the central nervous system. Therefore the innervation of muscles may be also a quite different one. The parallel or even diverging direction of the lines of vision, which accompanies the profound meditation, is indeed a relaxation, as far as the two m. recti externi⁵ are never contracted together. If we look upwards in the meantime, there is contraction of the rect. superior and obl. inferior, if to one side, the right for instance, there is contraction of the rectus externus of the right eye and of the rectus internus of the left eye. But looking straight before us, the two recti externi and recti interni may be considered as being relaxed. The active condition is the convergence. It is well known, that a blind eye, which is no longer moved inwards for the purpose of binocular vision, almost always deviates outwards, after a short lapse of time.

The innervation of the eye-muscles is a most interesting problema. It has been studied in my laboratory two years ago by a Russian, D^r. Woinow. His results are only published in Dutch, Russian and German.⁶ They are shortly reproduced in a paper, which I published, some weeks ago, p 13–15, of which I take the liberty to send you a copy. Probably a French abstract of this paper will be published, and I will allow myself to send you also a copy of that.⁷ You will see, that the innervation, regulated by the corpora quadrigemina,⁸ is always simultaneous for the two eyes, the double-eye of Hering.⁹ Now it never succeeds to produce by irritation of the centrum, contraction of the two recti externi beyond the normal abduction.— Of course, we can imagine that in the parallel position of the lines of vision both, recti externi and interni, remain in a state of contraction; but this would only represent an increase of the elastic forces in equilibrium, which has no object. At all event, if a slight contraction (more than tonus and elastic tension) remains, it will be the same in the mm. externi et interni,—neither prevailing, when the mind is lost in meditation. Just now, writing this letter, I find, that holding an object near to the eyes, I can, in alternating fixation, much more rapidly converge for binocular vision of this object, than diverge for binocular vision of a remote object: in the last case the double images approach even with diminishing rapidity, evidently because the contraction of the external muscles terminates here.¹⁰

It will not be necessary to remark that in every position of the eye the elastic forces of the contracted and relaxed (but extended) muscles are equal, being in equilibrio. Now I admit, that, without contraction by innervation the elastic tensions of the internal and external straight muscles determine nearly the parallel direction of the lines of vision (in normal eyes), and that, looking downwards, the direction becomes a little convergent, looking upwards, a little divergent. This difference corresponds to the fact, that we look generally downwards for seeing near objects, with convergence, and upwards, for seeing remote objects, without convergence. Indeed, it seems sufficiently proved, that the direction of the external and internal muscles, which changes a little with the mouvements of the eyeballs upwards and downwards, explains the indicated difference,—though I could not deny, that difference of innervation also is not quite excluded.

Allow me, dear Sir, to add a last observation: it is this, that the divergence, in question, is very slight, and certainly in some eyes des not or scarcely exist. Moreover, you will have observed, that I always speak of the lines of vision, not of the optic or corneal axes. They do not coincide, and the difference in different eyes is rather important. Now the expression depends on the direction of the corneal axes, not directly on that of the lines of vision. Where the lines of vision are parallel, myopic eyes seem to converge, hypermetropic (the inverse refraction of myopic) seem to diverge. If you wish to enter in this question, you find some remarks about it in my book: on the anomalies of accommodation and refraction of the eye, p. 248 seqq.—¹¹

I hope, my dear and most honoured Sir, to have proved, that I have the best will, to give you every information, you like, from the physiological department. It is a pleasure and an honour for me to do so. If now I do not succeed and if I fall in too great prolixity, Let me hope, that you will excuse me. Next time, I will try to be more concise.

I heard with the greatest pleasure that your health is now much better than it was, and that you are now getting on very well with your essay.¹²

Believe me, my dear Mister Darwin, with great respect, | Yours very sincerely | Donders

Utrecht, 1 April 1872.—

a tricennium after the 1 April 1572,—the most memorable day in our history, so nicely related, as You observed, by Motley¹³