To F. C. Donders   29 March 1872

My dear Professor Donders

In your letter about the eyes of a person lost in meditation, which interested me so much, you shew that the eyes are not fixed, & that the lines of vision even diverge; the divergence being greater, the more the eyes are upturned. You consider this divergence as passive, or as due to the relaxation “of the internal muscles”.¹ Now I am anxious to learn which are these muscles; & why, when all the muscles are passive, do these pull the eyeballs, & the more outwards the more the eyes are upturned? Are they stronger than the other muscles, so as to conquer them when all are equally passive or relaxed? I am the more anxious to understand this, from what Sir C. Bell of the rolling upwards & inwards of the eyes during sleep, fainting &c   He accounts for this movement by the four voluntary straight muscles ceasing to act as consciousness fails, & being then overmastered by the oblique muscles, which he says are little or not at all under the power of the will.²

Is this correct? I ask because I have heard that Bell was mistaken about the oblique muscles. But what most concerns & perplexes me is, that according to Bell, when consciousness begins to fail & the muscles of the eye are relaxed, the eyeballs are turned upwards, & inwards;³ where as you have shewn that when the mind is lost in meditation & the muscles are relaxed, the eyeballs are turned outwards, & so much the more outwards the more the eyeballs are upturned. If during incipient sleep or fainting the eyeballs had rolled outwards, the 2 cases w^d. have been harmonious. Now will you forgive me for being so very troublesome, & before long try to remove my difficulty?

I am now getting on pretty well with my essay on Expression, but I have been sadly delayed by ill health all last summer & Autumn.⁴

With cordial thanks for your former kind & to me invaluable assistance, believe me | my dear Professor Donders | yours very sincerely | Charles Darwin